Spoiler: Sources

The following sources have been used:

Spoiler: CR 11 highlight

First, a little talk about power balance. The Alien and its ally the Omen are meant to be fought several times over a campaign, pitted against 11-20th level PCs. Although the Alien alone has a minimal CR of 8, its mount starts out at 11. Even though the particular rules of this challenge tell us that the CR of the Mount is not accounted for when calculating the total CR of the villains, it seems a stretch of the ruleset to give the Rider a Mount more powerful than it is, in order to pump up power and bypass CR limitations. This is why from now on, I will assume that the Alien and the Omen will always come into play in the adventure with equal CRs. This is already supposed to add 2 to the overall CR of the encounter. However, PCs are expected to deal with monsters of a CR equal to the average level of the party four times per day, or monsters with a CR two higher than the average level of the party two times per day. As nemesises of the PCs, the Rider and its Mount can be expected to be a tad stronger than the average CR-equal-party level random encounter the party might fall into. Thus, even though the total CR of a battle against the Alien and the Omen is equal to the CR of the Alien +2, it is entirely reasonable to pit the duo against a party of an average level of the Alien's CR. And this is why, as the minimum CR of the Mount is 11, the lowest CR at which the PCs might encounter the bodak and the roc is 11.

Spoiler: How to play it as a plot hook

As powerful servants of the elder evil Leviathan, the Alien and the Omen try to further its agenda (as is, total destruction of all order and awakening the slumbering, deeply underwater, coiled-around-the-whole-planet monster) as soon as they can. Thus, the first time the PCs hear of the Alien might be when they are resting in a town, and popular rumor has it that a nearby thorp has been entirely depopulated overnight, without anything being stolen or damaged. The whole population has simply vanished. As the PCs go inquiring at dawn, they may glance a dark birdlike creature spinning away high in the sky, or fading in the fleeing night. Once they're in place, a quick Search reveals the location of the bodies - or what remains of them : all of the heads have been savagely severed and disposed at the bottom of the well. Strange runes are painted with blood all around the well's edge. As it turns out, these runes are foretelling the soon to be awakening of the Leviathan.
As the chase starts, the PCs are closing in, and the more desolated small towns they find, the more distinctively they can see the ominous shape of a titanic bird, each time flying away from them but each time closer. The puzzling massacres always occur by night. Finally, they arrive just in time to see the Alien finishing the eradication of a last small thorp. A terrified farmer tries to flee, but suddenly dies where he stands without a single sound. Carelessly looking at the PCs,who must immediately save against its Death gaze, the Alien mounts on the Omen and, still without a sound, gesture or display, flies away in a giant leap. This time, the bodak has not had enought time to pile on the heads into some waterpond and draw the unholy runes of the Leviathan.
Now that they have seen it, the PCs can then start to foray for various informations about the creature. They discover that it does not content itself with killing every living form it can encounter : it also makes sure that the souls will never be able to come back. Its sinister mount Coup de Grâces the victims with its Harvester of souls feat, then severs the head and eats the body. Tales relate that although headless bodies have sometimes been recovered, standard resurrection magic mysteriously fails to bring them back. The servant of the Leviathan is a (un)living foreshadowing of the Apocalypse to come with the awakening of the ancient monster. Always silent, always still, it seems to cause death and desolation by its mere presence. Wherever it goes, sorrow follows. It is Death incarnate.
The PCs finally manage to confront the Alien in the last few hours before the sun rises, accompanied by DMPCs of some sort: terrified city watch, Crusader of Pelor, etc. These DMPCs, of course, are only here to get worfed by the Alien, as well as to show its modus operandi to the PCs. The bodak finally realises that those pesky cockroaches might actually prevent him from advancing the rise of the Leviathan, and let the party catch up with it for a confrontation.

When all the NPCs are dead, or one of the PCs has fallen to the bodak's Gaze, or if the Rider is about to be taken out, a sudden out-of-season blizzard sweeps over the battlefield. It is a consequence of Leviathan stirring in its sleep, as its minions sow death and destruction to herald its arrival. Seeing this as a sacred message, XXX departs at the moment, understanding that it has only lost too much time dealing with these earthbound ignorants. The Ritual of Slumber must be found, wherever it is hidden. Moreover, the sun is about to rise, and bodaks are tickled by its fiery bite.

Spoiler: The duo in battle

Here is the first trick of the villains: the Omen swoops down on one of the DMPCs, attacking it with Flyby Attack. It attacks for a +16 to-hit while pumping up +5 Power Attack, and reliably hits, since it always performs touch attacks with the Restless Prey template; these attacks also ignore any DR. It deals an average of 27 damage (it also adds Cha to damage with the template), which is not too shabby: an 11th level rogue with Con 14 has 63 HP. With the Snatch feat, it automatically initiates a grapple (+24 modifier, + the leftovers of the roc's BAB) and then carries on its move action (per the Flyby Attack) and gets some 200ft away from battle. Indeed, the Air Heritage feat grants it +30ft to its base fly speed, which is then doubled by the Restless Prey template. In the process, the Omen deals a negative level with no save by the mere contact of its talons. Its Fear aura also ensures that, save for fear immunity, the target is either shaken (on a successful save) or cowering (on a failed save, it panicks them is caught in the talons of the roc, making it effectively cornered). At any rate, -2 to saves and attack rolls, in addition to the -1 from the negative level, which makes the grapple check easier, as well as any psionic powers that would follow; a cowering guy can take no actions for 20 rounds or so, so it is done for. The save DC is 22, so not too bad when an 11th level fighter will scramble for a +7 Will save. To top it off, the Abominable Form feat, taken by each villain, forces two DC 19 Will saves against shaken for one round. On the same round as a free action, the roc can let its prey fall. 200ft with a 45° upwards inclination, the maximum allowed by average maneuverability, gets us 140ft aboveground. As a falling PC gets down 150ft in the first round of fall, the character takes 14d6 right away, so 49 additional damage.

The PCs, however, may have Freedom of Movement (FoM), Death Ward (DW), Fear immunity (FI) or Fly. But it is unlikely that all of them are protected by these spells. The one who lacks FoM gets Snatched away.

The one who lacks DW gets Death Gazed away: save DC 25, ouch. An 11th level Cleric with Wis 20 has Will +12, so death is more likely than survival. This is what a Death Rider is about. So the party will have to fight with closed eyes, giving concealment or total concealment.
As for the psionic side of things, Wild Surge (WS) +2 and Practised Manifester give us Manifester Level (ML) 9, only 2 less than full manifesting for a CR 11 Psion. Only 18 power points prove our downfall, though. Vigor for +20 HP at ML 4 on the first round while the Omen Snatches away from battle, then two Energy Rays (ER) for 40 damage on average at ML 9 on the following turns, no save, and +10 to-hit on the touch attack. The Alien readies an action to Energy Ray the next target to be Snatched just before it is seized by the Omen. The ability to to damage to objects with a Sonic ER grants some versatility : Bet that +1 Disruption heavy mace doesn't have nearly as much hit points as its paladin - or the roof behind the feets of this Daring Outlaw. Mindlink allow it to communicate with the Omen even though the roc does not know any language.
And here is the second trick: per the SRD, the Alien can make a Concentration check with a DC of 15+power level to suppress all displays of the power. So the PCs -and the players too !- can't understand what happens. Objects explode, characters suffer invisible burns or electrical shocks, but they can't see exactly what is going on. Very flavorful for It which rides before Death: everythings gets destroyed seemingly by its mere presence !

The bodaks thus pumps up Concentration. Ride is high enough to auto-succeed on DC 15 checks (with a masterwork saddle), which are the highest relevant checks it can be brought to make. The roc pumps up detection skills, to more benefit of its +8 racial bonus. It can already beat the DC 20 for detecting an immobile, invisible creature.

The villains don't have that many HPs, nor saves, nor AC. But the roc is so fast that is can begin its turn 100ft away from battle, shuffle right through the melee, damage a character on the way, get the bodak to fire off a readied ray, and flee 100ft away again, consuming the last 20ft of movement to make a U-turn (average maneuverability). PCs can ready actions, too, but melee characters, even fllying ones, will never ever land a full-round attack, and neither will short-range blasters, precision-damage (with Truedeath Crystals) dealers and the like.

Spoiler: CR 15 highlight
Spoiler: How to play it as a plot hook

As the manifestations of Leviathan's return grow stronger, the PCs might get distracted in their pursuit. Windstorms settle and disappear at random, the tide does not follow its normal cycle anymore. Many people need their help. Thus, by the time the PCs meet the Alien again, they have dealt with other subplots, and gained a few levels. They are not the only ones.
The party was foraging into the royal library, looking for a copy of the Ritual of Slumber; which has the power to put the Leviathan to sleep again. No sooner have they entered than they can glance the Alien entering the library by the opposite side after having demolished the wall, looking to destroy said book. A trail of headless corpses in the street behind the hall marks the passage of the Emissary of Death. The party's casters haven't prepared a particularly high number of Freedom of Movement, nor Heal, nor Death Ward. The battle will at first occur among the shelves, both barricades and weapons. The Omen doesn't fit through the hole in the wall: it will spend the first few rounds to further demolish it, in order to get in. After a tense battle during which the library is set on fire, the Alien manages to escape with just the right page ripped from the book, intent to destroy it in a special dead magic zone to prevent Limited Wishes to repair it. The party follows, and a furious aerial race begins.

Spoiler: The duo in battle

The basics of this CR is the apparition of Dispel Psionics. It allows Snatching, Fear auras and Death Gaze to remain relevant despite the ever-increasing presence of DW, FoM, FI, and the like. The Alien can deliver a ML 13 Dispel Psionics (DP), for a 1d20+18 dispel check. Against a CL 16 (15 +1 for a Ioun stone, for example) DW, we get a 60% success chance. An area DP has high chances to switch off the DW or FoM of two or three party members, but can't affect items such as Ring of FoM.
Moreover, we now get WS +3; so manifesting said DP only costs us 10 power points (pp). In case of psychic enervation, we only lose 7 pp. Once per day, the Third Eye: Clarity negates the Daze. But for a villain, once per day is just like once per encounter anyway. This is also the reason that explains that though the Alien only has 82 pp, it can remorselessly eat them all in a few rounds of nova, whereas a PC could clearly not.

-Once DW is removed, Save DC 29 Death Gaze is very powerful. A 15th level Fighter with Wis 12 and a +4 cloak of resistance has Will +10.
The bodak also hits very hard with Surging Euphoria +1 (SE +1). +1 to saves and to-hit is nice, +1 to damage rolls (not excluding ability damage rolls) is great. ML 14 fire or cold Energy Ray deals 14d6+28 damage. Add in another, Quickened one : 8d6+16 damage. So, 114 in total. No save, only SR. Those with good Will saves to resist Death Gaze, like Clerics with Con 16, have 116 HPs. Gloups. Crystal shard can hit characters with very high touch ACs since it allows no to-hit, no save, no SR, only DR.
Ego Whip augmented at ML 11 and boosted by SE +1 dishes out 3d4+3 Cha damage, or 10.5; this one-shots any Cha-dumping character who fail a DC 22 Will save. The save is only average but remember that the duo of villains impose three successive saves against being shaken by their mere presence. Once the Fighter is helpless with Cha 0, the Omen Coup de Grâces him right away (since the faster of the duo always delays its turn to act just after the slower), using a combination of 5ft-steps, Sudden Leap and 15ft-reach to get to him and still have a full-round action. With Harvester of souls, hello to temporary HP and goodbye to anything but Wish to bring the PC back - getting a scroll of one can sustain a sidequest. If the fight happens midair, the PC falls right away, which is hilarious !
A Quickened Vigor gives us 40 temporary HP. By the way, move actions are useless to the Alien but to fuel its Psionic Meditation feat, ensuring that it will get two powers each round. The advantage of having a mount.
Concentration is still pumped, while Intimidate is for Cha synergy, +5 bonus from Deformity feats, and fluff, not fight: The Rider of Death's reputation sends ennemies flying ! The Intimidate+Mindlink trick (see the Murky LAW section) is now online. Psicraft is a prerequisite for Psionic Meditation.
Evil's Blessing pumps up the saves by a +6, and once again for a villain, five-rounds duration = permanent.

-If a FoM is dispelled, then go Snatch strategy ! The Alien readies an Energy Ray to unleash on the prey once the Omen lets it drop, just before the Omen Dive attacks to finish it off. Dealing nonlethal damage on purpose can allow for a Coup de Grâce the following round.

-If PCs can't be singled out, the Omen is now a level 4 Unarmed Swordsage, unarmed damage 4d6. It uses very offensive and debuffing maneuvers, like Disrupting blow. However, the Omen needs Mind Over Body and Moment of Perfect Mind as prerequisites, though it doesn't have ranks in Concentration. As an airborne fighter, the roc does not touch the ground, and so Stone Dragon maneuvers can't be used. Wisdom is too low for Shadow Hand maneuvers' save DCs to be high enough, and teleporting effects are useless since they don't teleport the Alien with them. Setting Sun impairs but does not kill ennemies, so the Omen does not learn from this discipline either.
Typical readied maneuvers : Pouncing Charge, Swooping Dragon Strike, Inferno Blade, Sudden Leap, Dancing Mongoose.
In Leaping Dragon's stance, the roc applies Weapon Focus to unarmed strikes, and Wis to damage. On any character with touch AC of 16 or less (about touch attacks, see the Murky LAW section), a combination of Pouncing Charge and Dancing Mongoose statistically dishes out 225 damage (no DR). This is 222 for Inferno Blade + Pouncing Charge -readied again through Sudden Recovery- assuming no fire resistance on the PC. What is great is the opportunity to distribute the iteratives amongst the PCs, thanks to the 15ft reach and a 5ft step: more negative levels dealed, and the roc can outright kill a PC while severly disabling another one. A +91 Jump modifier (thanks to 220ft fly speed, see the Murky LAW section) allows for an auto-stun with Swooping Dragon; once again, a stunned flying character just falls 150ft, meaning if a comrade wants to rescue it, it will lose at least a move action to get to its altitude - if it can make it at all ! The Jump check involved also allows for a complete 100ft repositioning without any AoOs. Sudden Leap provides the same benefit, or allows for another pseudo-Pounce. All in all, the roc has something to do with swift actions for 5 rounds, maximizing action economy profit. Note that, for added mobility, strikes like Swooping Dragon or Disrupting Blow can be made as part of a Flyby Attack, or a Snatch attempt - let's stun the Snatched away Psion so he can't Dimension Hop away on its next turn.
Here again, Evil's Blessing grants +3 to all saves. Hearing the Air informs you that invisible ennemies are near, then Clarity of Vision skill trick pinpoints them as a swift action. This makes up for not pumping perception skills. All the moving around by the roc is facilitated by a +23 Tumble modifier, always handy for AoOs. 1 rank in Jump allows not to fall on one's butt at the end of a successful Jump check, as normal for untrained Jump use.

The duo now has great saves, and is still enormously master of the battlefield with its speed. Lack of SR is okay since the main spells that affect undead, don't allow saves but allow SR are blasts, which means casters have to fight in a suboptimal way. Concealment from opponents closing their eyes because their DW got dispelled helps immensely. However, even with Vigor, their HPs (107 +70 temporary, and 149 for the Alien and the Omen) and their ACs (22 and 23) are ridiculous: if the hit-and-run Snatching fails, full attacks can occur; and then a well-placed moderately-optimised pounce or True Death Crystal Sneack Attack or Orb of Energy or Disintegrate...well, if the villain can't be beaten, what is the point of using it ?

Spoiler: CR 20 highlight; sweet spot
Spoiler: How to play it as a plot hook

Now, this bodak and his roc are no laughing matter. When the Alien reaches a new city, at first the inhabitants will spot the now infamous shape of the Omen, rapidly growing in the stormy sky. The Alien even dares to appear under the bright sun, bolstering itself with false life against its sting; but the bright sun never lasts for more than a mere minutes before a blizzard or a sandstorm unnaturally strikes down. Citizens flee before the Rider of Death. City watch gathers, clerics leave their temples and incantate, warded against death magic. As the duo closes in, everyone can see the wrenching deformation it brought upon itself. Defenders flee. Others close their eyes and wait for the clerics to command them. The Omen swoops over the fortifications without attacking, but its mere presence sends dozen more running for their lives. At the moment the roc flies above the clerics, an invisible, insensible wave of psionic energy washes away their divine protections. Doing a swift aerial U-turn, the Omen comes back, and the Alien looks at them. Some die on the instant, others survive, but their brains has not yet processed that they are still alive than they suddenly turn into piles of smoke. Turn Undead, Undeath to Death, Holy Word, nothing seems to affect Death that Rides. Waiting orders who don't come, soldiers glance around and meet their fate. Defenders suddenly freeze while they are drawing a sword or a scroll, or decide to stab their own eyes out. Some others suddenly fall into a dreamless catatonia. A few number is consumed by invisible fire, implodes with inaudible destructive harmonics, is frozen to death by intangible ice. The last survivors, trying to hide between a wall of shields, see huge bloody holes appear out of nowhere in their comrades, just as if some blazingly fast yet invisible blade had perforated them from side to side. Only then does the Omen land on the parapet, and methodically decapitates the dying ones, while the remains of the dead abruptly poof out of existence into piles of dust, one at a time. The Alien has not made a single move. The duo has not uttered a single sound. That is a Rider of Death.
But despite its best endeavors, the PCs have managed to unearth the final copy of the Ritual of Slumber, and they are intent on carrying it on no matter the cost. All around the world, sunken mountains randomly rise, coastal cities are submerged in a matter of minutes, inland volcanoes appear out of nowhere and devastate entire capitals. Leviathan avatars have been spotted: the final sign of the resurgence of the Elder Evil. As a forewarning of what is to come, mortals have discovered with horror than anyone dying within a large radius around the avatars has its soul destroyed to pieces by the seeping essence of the Leviathan (Elder Evils p.69). Even gods can't bring them back. This is what the Alien and the Omen have been emulating all this time.
The PCs are now engaged in the temple of the Surging Spine, on the very back of the stirring creature. The titanic bony protrusion is hollowed out in immense cavelike chambers. Two weeks ago, the party has cleansed the place of five powerful bodak priests, among which the mentors of our Rider and its Mount. Now, rested and equipped, the PCs return to finish the job.They try to access the lower part of the temple to find the ritual room; the door has ominously closed behind them. But as they forage around the room, surrounded by the same runes that the Alien used to paint everywhere, a Time Hop ends. The Alien and the Omen were waiting for the party, and had hidden themselves in time itself to surprise the intruders.
Since any dead character during the fight is supposed to never be able to resurrect, this climactic battle is a good moment to bring the campaign to an end, whether the Evil is finally put back to sleep or manages to fully awake and unravel all that is.

Spoiler: The duo in battle

Many powerful additions to the Alien's arsenal appear. Touchsight is as useful in-battle than out, to prevent the PCs from avoiding the encounter. Time Hop gives a lot of flexibility, as shown above. One cubic meter of air weighs roughly two pounds. Why not make all the oxygen in the closed room disappear at once ? Death Urge is a Will-based save or die very thematically fitting: people may run away and avert their eyes, but all of a sudden they start to stab themselves ! No escape is possible. Moreover, in an aerial battle, an affected PC might chose to voluntarily crash to the ground, for extra fun if not extra damage (see the Murky LAW section). Ectoplasmic Cocoon is a Ref-based save or die, since it makes people helpless, that is, Coup de Grâced on the following turn. Moreover, as display is hidden, the target seems to suddenly freeze for no reason. Visual, but also material display is simply not there, so even probing the air around the character won't reveal the cocoon ! Add in a Fort DC 33 Death Gaze, and we have save or dies for all saves. WS +4 allows the bodak to get full ML ! Thus, it can pump the save DC of all of its powers to 29. Any 20th character is likely to have a bad save not exceeding +16.
But here are the big guns, the reason why this is the sweet spot. Schism, Disintegrate, SE +2. SE +2 allows a max boosted Ego Whip to dish out 5d4+10 Cha damage, or 22,5. Wilders are well-known blasters: Energy Ray can pump up to 20d6+60, or 130. Disintegrate, while allowing Fort save, delivers 220 damage, and leaves no corpse, so hard time resurrecting that, even far from a Leviathan Avatar. But between Schism and Quicken spell, the Alien can really go nova. 182 pp allow for three rounds of that. Start the encounter with Schism, Vigor and Touchsight on. Get a surprise round with the Time Hop trick. Then, Quicken Electricity (for +3 to hit, total +20) Energy Ray the 180 HP rogue with a WS +4: 49 damage. Third Eye: Clarity away the eventual Daze. Now, Schismed Energy ray, Fire rather than Electricity since SE gave us +2 to-hit; at WS +1 to be safe, 61 damage. Finally, standard action area Dispel Psionics, dispel check 1d20+20 this time, no need to WS.
Following round: if the Omen is swooping, ready a max power Disintegrate, Ectoplasmic Cocoon or Death Urge, plus a Schismed Disintegrate (for readying two manifesting at a time, see the Murky LAW section) and fire them off "when the Omen gets at less than 50ft from at least one PC", for example. During your turn, manifest Quickened Time Hop on the ground beneath a PC's feet, or Vigor to heal temporary HP, or activate Evil's Blessing: +9 to saves (and another +2 from SE). If the Omen is in the thick of it, targeted Death Gaze, or maxed out Ectoplasmic Cocoon, or Death Urge, or Disintegrate (if DW did not get dispelled) a PC to oblivion. And fire off two more WS +1 Disintegrate for 121 damage each, one to finish off the Rogue, one to get the Bard at less than half HP. So, if all works well, the Alien has many options to down two PCs per round.
Intimidate is maxed out, still for fluff, and Bluff comes along for the synergy. How can one guess the thoughts of a never-speaking, never moving undead ? It keeps enhancing the flavorful Intimidate + Mindlink trick too.

The Omen is equally a terror to behold. Typical maneuvers readied: Time Stands Still, Feral Death Blow, Raging Mongoose, Inferno Blade, Pouncing Charge, Moment of Alacrity, Swooping Dragon Strike. Typical Stance: Pouncing Dragon's Stance.
If the roc has he opportunity to Snatch away a non-FoM foe, it does this: Start a move action, initiate Swooping Dragon's Strike in the middle of it thanks to Flyby attack. Deal a negative level in the process. Start a grapple with a +43 modifier. Fly 100 ft away. If this maneuver is already expanded, just use a Swift Recovery to gain it again.
In head-on battles, the Omen is now incredibly mortal. 4d8 unarmed damage. With a +36 Tumble modifier, making 10ft -instead of 5ft- steps as a DC 40 Tumble check (OA rule) is almost guaranteed, the roc can thus reach out to 25ft around. Multiattack is very useful, since full attacks are made very often. Power Attack -12, Inferno Blade and Time Stands Still gives us +16/+13/+13/+11/+6/+1 touch attacks, twice, dealing a statistical damage total against touch AC 18 of 530 damage, of which roughly 220 is fire damage. There is largely enough to kill two PCs with a weak touch AC, as it is very likely that two PCs at once will be within reach. PCs with better touch AC have less HP, so it evens out. What a sweet spot !
Other options include Feral Death Blow (death against Fort DC 31), followed by a Quicksilver Motion to get out of melee, or Raging Mongoose Pouncing Charge. The latter deals respectively 335 damage agains touch AC 18, one-hit ko on a frontliner; or without Power Attack, 242 against touch AC 30, one-hit ko on the rogue.
Pouncing Dragon's stance still enables the three Discipline Focuses to kick in. Flame's blessing grants fire immunity. Hunter's Sense provides with overland Track; Hearing the air allow for a +31 Listen check, assuring the evil duo to roll for initiative at long distance; moreover it provides redundancy with Touchsight and Clarity of Vision to better foil close invisible opponents.
Tumble gives us 10ft steps and avoiding of most AoOs, in particular when used with Desert Tempest to escape the heat of the battle while damaging everyone.
The roc's saves are high thanks to Evil's Blessing and Apostate, but it still has ridiculous HP and ACs, and undead immunities are easily bypassed at CR 20, this is why...

...as a rule, the Omen gets away from the PCs as much as it can and only hits with Flyby Attack + a strike maneuver, or hits and run with Quicksilver Motion. As much as possible, the Alien singles out foes by dispelling the FoM, and the roc Snatches them 100ft away; even though, if not Stunned, they can Teleport out of a Grapple at this level, they will rarely flee 100ft in the process, so the Omen can catch up (while the Alien makes them helpless as shown above) and Coup de Grâce them while they are still isolated. The big guns who kill PCs on the spot but let the other ones at close range should only be used if they let no more than one PC at close quarters. The duo has high saves, but its HP and AC are so miserable that it needs to avoid facing too many direct damage dealers as much as possible, as it can be brought down in one round.

The duo too, however, has the potential to wipe out an entire 4-characters 20th level party in one round, and the PCs know that, since they have already fought twice with the villains. The battle can very quickly tip either way. The players will probably be incredibly cautious in their play, as death is definitive; this will give a very gritty fight. Or they will be reckless and sacrifice themselves beyond any resurrection for the cause of good: what a blazing end to a campaign ! Either way, roleplay and fun come out winners.

Spoiler: Murky RAW lawyering

Here we get the following issues looked at.

  1. Schism and Death Gaze

    Schism states, per the SRD: "Your new “second mind” does not control your body physically but is free to take one standard action in each round if the action is purely mental (such as manifesting a power)."
    Gaze attack, on the other hand (still per the SRD): "A creature with a gaze attack can actively attempt to use its gaze as an attack action. The creature simply chooses a target within range, and that opponent must attempt a saving throw."

    So, can the Schismed mind use its standard action to target a Gaze attack ? Nowhere in the Gaze attack text it is said that the creature must physically turn its head to make eye contact with its opponent, so by RAW that means that targetting a Gaze attack would qualify as a "purely mental action". However, it doesn't seem very logical that the creature does not have to move even its pupils to target the Gaze, so I suppose this is up to you, the DM.
    The consequence is not that marginal. Its means that at CR 20, the Alien can use Psionic Dispel at ML 16 with its normal mind, and so without having to Wild Surge; and gets a Dispel check of 1d20+20, instead of 1d20+18 80% of the time, and being Dazed the remaining 20%. Then, its Schismed mind could Death Gaze the Dispelled character. Generally speaking, having the Schismed mind performing a Gaze attack instead of manifesting bypasses entirely the -6 to ML it gets.
    I'm on the RAI side on this one, but this can be used a a leverage factor to enhance the Alien's power in a pinch.

  2. Jump and fly speed

    By the SRD: "Your Jump check is modified by your speed. [...] If your speed is greater than 30 feet, you gain a +4 bonus for every 10 feet beyond 30 feet." It does not say what kind of speed. Once again, by RAW, Fly speed qualifies as a "speed" for this purpose. Given the 220ft fly speed of the Omen, it nets it a +76 bonus to Jump. By RAI, it seems quite logical too, as for a flying creature, a jump is no more than a flight of very short duration.
    Why does it matter ? The save DC of the Swooping Dragon strike to avoid Stun is the result of a Jump check. Moreover, the Sudden Leap boost allows to move the result of a Jump check a a swift action. The Feral Death Blow strike needs a Jump check that beats the target's AC bonus. And if the fly speed bonus is ruled out, then the Roc only has ranks +8 (for its 40ft land speed, +2 from Air Heritage feat and Tumble synergy) or ranks +12 (if the Alien uses the "Spur Mount" ability with a DC 15 Ride check to give it +10ft speed for one round) as a Jump modifier. The Leaping Dragon's stance directly gives a bonus to Jump expressed in feet, and so it can't be added to the result of the Jump check.

    This means that the roc now has to pump up Jump and to give up either Spot or Listen, which I find a pity. I'm on the RAW side on this one.

  3. Force attacks and unarmed strikes

    The Restless Prey template stipulates : "A restless prey creature's natural attacks are composed of pure force and remain the same as those of the base creature, except that they are force attacks and they ignore armor, natural armor, and shield bonus, as touch attacks do. A restless prey creature's force attacks are not subject to damage reduction."

    The Rules of the Game: Unarmed Attacks (Part Two) article holds that "A creature can choose to treat its unarmed attacks as its primary attacks and its natural weapons as secondary attacks. (This method is normally used to add weapon attacks to a natural attack routine.) The creature must make all unarmed attacks with its primary limb, which prevents that hand from being used for a natural attack such as a claw or slam."

    Now, by RAW the creature that uses one of its primary limbs to deal unarmed strikes doesn't get Force unarmed strike, since they are no natural attacks. In the case of the Omen, this means it will forgo one of its talon attacks, and gain four ordinary unarmed strikes plus one talon and one beak Force natural attacks.
    But common sense dictates that, as the unarmed strikes are made with a Force talon, and the other Force talon deals Force natural attacks, the unarmed strikes must be Force ones too. It would make no sense that, simply because the Roc uses its two talons differently in an attack, one of them suddenly ceases to be made of Force. In which case, one wonders out of what it is made, since the talon is not sheathed by a Force effect, or similar : it is physically made out of Force.

    The difference is of great importance. It dictates whether the Omen can dish out, in a full attack, 2 touch attacks ignoring all damage reduction, or 6. Or 10 with the Raging Mongoose boost. Personnally, I am in favor or RAI; but generally speaking, as the DMs has the final say and the DM is you, this matter (as with all others) easily become another leveraging factor, that you can play with or not depending of how the fight turns out.

  4. Death Urge and flight

    Funny one. The Death Urge power states that "A subject close to an immediate and lethal hazard such as a cliff or a fire might hurl itself off the cliff or into the fire instead of striking itself with a weapon." At high levels, all of the fights will happen in the air, with flying PCs and fllying villains. So, a flying PC affected by this power might simply dive to the ground at maximum speed, as it is standing above the void, which certainly stands for a hazard both immediate and lethal. A character that stops flying falls 150ft the first round. So the Alien has to move the fight at just the right altitude, and the PC gets to the ground and 15d6 damage. The advantage of this compared to the simple "I attack myself at full force" is that if a fellow PC wants to come in and dissipate the spell/heal the guy, it will have to get to the ground too, losing at least one turn in the process, and then get up again, losing yet another turn.

    I'm confident that's pretty RAW, since if the character would "hurl itself off the cliff", there's no reason it wouldn't hurl itself in the void. The only reason that could go the other way round is that the PC is very likely to hurt itself more by an auto-critical hit full attack that by crashing to the ground, so the player might feel spoiled if this is not what happens and it makes the PC out of help's reach. Once again, usable if the bodak needs some breathing room in a fight, whether your players like it or not.

  5. Schism and readied actions

    So, we already pointed out that the standard action granted each round by Schism can only be a purely mental action. Now, what does the SRD tell us about readying actions ?
    Here you go. The SRD states that you can, for example, ready a counterspell. Then, when the triggering condition occurs, if you "can cast that same spell (are able to cast it and have it prepared, if you prepare spells)", the counterspell goes off. Emphasis mine. Here, the SRD shows us that the availability of the readied action is not checked when the action is readied, but when the action is carried out.
    To my understanding, this means a Schismed mind could use its standard action to, say, make the left eye of the character wink after one full minute has expired. But then, after one minute, the following sequence would start.

    1. What is the readied action ? Make the left eye wink.
    2. Is the triggering condition met ? Yes.
    3. Is the Schismed mind able to make an eye wink ? Since this is not a purely mental action, no.
    4. Is the readied action carried out ? No.

    The Schismed mind would not carry out the action.
    The Rules Compendium mostly only reformulates the SRD, however it explains that "Think of a ready action like a basic computer program: if this happens, then this occurs." This is why it seemed accurate to me to express the above sequence as a sort of computer algorithm.

    As a consequence, as manifesting a power is a purely mental action, I believe that by RAW a Schismed mind not only can ready the manifestation of a power just as well as he can ready an action to scratch the nose of the manifester, but it also will manifest it when the right conditions occur, whereas it won't scratch the nose. And I think it seems quite logical and not too much far-fetched, either.

    The consequence ? When the Omen makes a Flyby Attack on the PCs, the Alien can ready two manifestings instead of one, once it gets Schism. One of those powers, at ML -6. The difference in power is minor, but not insignificant.

  6. Intimidate and Mindlink

    The use of the Intimidate skill out of battle allow you to turn any character to "friendly" attitude for a short duration, if you beat the opposite skill check. Now, "friendly" means that the actions the friendly character can take are "Chat, advise, offer limited help, advocate". Does this mean that a friendly character can be considered a "willing target" for a harmless spell or effect ? The SRD is not of great help about what "willing" means, either. So, without a clear definition, it is hard to find some RAW telling us that yes, a friendly character can be considered willing to be targeted by a spell that doesn't make him act more than "Chat, advise, offer limited help, advocate".

    This means that by RAW it is unclear whereas a character bullied by Intimidate is willing to be targeted by a Mindlink by the Alien. The bodak just needs information; it could perfectly vocally ask the Intimidated NPC, and, as a friendly character, the NPC would answer. But vocally expressing oneself is far below the condition of the Rider of Death; this is why it prefers to coax its victims into accepting a Mindlink, before telepathically questioning them.
    If the PCs stumble upon such a situation between the bodak and an NPC, the DM can of course let this fly by Rule of Cool; however, if a PC happens to be the target of such a manipulation, the player might want a more proper RAW frame for his character, hence the need for this discussion.

    I ultimately think that this technique is at least RAI pertaining to what the Intimidate skill allows; and to satisfy an unhappy player, as a DM I would allow the PC an opposed Will check to let the PC momentarily take the upper hand in the telepathic discussion and manage to force the bodak to answer one question. At any rate, the flavor this technique adds is worth the trouble of justifying it, so I would allow it to work. And even if another DM doesn't, it ultimately does not make a great difference in the Alien's power or in its depth as a villain.