Spoiler: Snapshots
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ECL 5
"You call this an 'open-and-shut', just because a dozen witnesses tell you the victims stabbed each other to dead? You can't trust people, sergeant. There's not enough blood at the crime scene, too many fatal wounds, inconsistencies in the coroner's report. Look at this: a recorded sale of black onyx, two weeks before the crime, by a mutual acquaintance of the victims. Do I need to spell it out for you?"
We're already an extremely capable investigator at low levels. Investigate lets us make Search checks to find and analyze clues at crime scenes. Research lets us navigate archives, libraries, filing systems, and more to retrieve key information, which we can then cross-reference with earlier findings or each other to find discrepancies.
Our skillset lets us sneak around unseen, chase rumors, interrogate witnesses, craft disguises, forge documents, decipher scripture, and even provide the odd bit of knowledge from time to time. Our Sense Motive is good enough for most purposes, but 1/day Zone of Truth helps out against those Naberius-binders, Beguiling Influence warlocks, or Motive Charisma Marshalls.
Master Inquisitive gives you a contact, which 'should be developed by the DM'. Thus, I'm not going to make any mechanical claims about this character, save for the assumption that they're vaguely useful in some way. However, I'd argue that in most cities the average 3rd-level warrior is part of the guard, so it seems reasonable to ask for a contact there.
The bad: Ilias is nearly useless in a fight at these levels. Poisonmaking hasn't come online yet, and neither have alchemy or UMD. Use your skills to stay out of combat, and if some face needs beating, don't do it yourself: gather (or forge) evidence of their crimes and pass it on to your guard contact.
If forced into combat, use some disposable alchemical items to escape (smokesticks are an easy way of gaining total concealment), then circle back later and analyze the place for clues to figure out what you're up against.
ECL 10
"Grevel Umber? This envelope contains a detailed record of your recent 'activities' in the Shades; don't ask how I know about them, but feel free to verify the truth. And before you reach for that blade: a copy is already in possession of the guard, set to be opened if I don't retrieve it today. Now, don't worry, I'll gladly take it out of their hands... for a small price, that is."
Remember when Ilias was kind of bad at combat? Well, now he's got poison use, 3rd-level spells from a great list, alchemical toys, 3d6+10 sneak attack, two attacks per round, and a death attack with a great int-dependent DC. I suggest switching to a poison ring as your main weapon (no sense in abandoning your stylish swordcane, though), as AC starts getting rough to hit. Our older pickup of Shadow Blade Technique is nice for ensuring an important hit actually connects: it's a bit embarrassing to spend three rounds setting up a Death Attack and whiff.
The spells are great: two different Invisibility spells set up sneak attack and help out with stealth, Vigilant Slumber protects us from a messy nighttime demise, Ebon Eyes helps us with darkness, Arrowsplit is great combat damage, Enduring Scrutiny lets us keep tabs on blackmailed victims, and Alter Self is great from a combat or infiltration perspective. Instant Search helps with qualifying for Unseen Seer and synergizes with our maxed-out Search.
Unseen Seer gives us the skills we need to enter Cerebrex next level, as well as this little spell called Guidance of the Avatar. Your second-level spell slots are now tactical skill nukes. We also get Silent Spell as a bonus feat, which is actually kind of neat for a stealthy caster.
We also awaken our dragonmark's powers, giving us 2/day Mending. It's kind of underwhelming, but hey, ceramic alchemical/poison vials are 1 GP a piece, and if you can glue them back together after they break (on someone's face), then that's a neat way to save some money longterm. Also, you can break wax seals and fuse them together again, or use a hacksaw on a chain/bar/manacle and mend the break afterwards to leave no trace, so this little SLA might come in handy more often than you think.
ECL 15
"Ah, apologies ma'am, no need for that language, I just applied a routine magical safeguard. Yes, the anger you're feeling is a natural side effect, it'll be gone in a minute. Now, if you could just repeat what you were telling me a moment ago, you'll be free to leave..."
Cerebrex is online, and we immediately start getting boosts to our Research/Investigate rolls through Enigma's Bane. Investigate specifically calls crime scenes 'puzzles', and Research mentions how it helps you navigate codes: the same language is used by Enigma's Bane. The ability might also come up in different circumstances, but at least now it's guaranteed to be useful.
What's also really cool is Aggression Mastery. At these levels, enemies have long had access to Glibness and its lesser cousins: optimized Bluff checks might range in the sixties at these levels. Not only are we incapable of seeing through a well-prepared liar,. The traditional solution to those issues is 'max out CL and hope your dispel breaks through', but that still leaves you to contend with the nonmagical bonuses, which tend to be sky-high as well (and at any rate, our CL is only 7 and we lack any dispelling).
Instead, just anger someone to the point they can't lie anymore! Rage specifically disables the use of 'charisma-based skills other than intimidate'. Someone in a Rage specifically cannot use Bluff, making their carefully accumulated bonuses to the skill worthless. It's a bit unclear how this translates on the mechanical level: either they physically cannot speak lies, or they can speak lies but everyone automatically succeeds on the Sense Motive to pierce them. Either way, we can safely verify the truth of any word someone speaks, even if they're likely to be angrily yellin. Obviously, the ability still has all its regular uses as a buff on martials and a debuff on spellcasters, and stealth makes the latter use case a bit more viable for us.
The good stuff doesn't stop here either: Symphony of Nerves grants a small but notable boost to dex, which we're set up to take advantage of slightly better than most characters thanks to Weapon Finesse and Sneak Attack (flat-footed targets are always valid SA targets, boosting your initiative makes more enemies flat-footed). Unlike other Cerebrices, we actually have dexterity-based skills, so that's another way we're set up to take advantage of the ability. Knowledge Devotion is another offensive boost that appreciates our investment in Knowledge skills, and we grab Collector of Stories to go with it. Magical Appraisal, our other skill trick, benefits from our maxed-out spellcraft by giving us the odd free Identify.
Telepathic Immunity lets us resist Detect Thoughts, which is an incredibly powerful spell in social/intrigue situations. If your DM likes open-ended readings and thinks it blocks Mindsight, our stealth-loving self certainly appreciates that (I do, however, not assume that this reading is RAW).
Cerebrex advances our casting by two levels, granting us 4th-level Assassin spells. Dimension Door and Greater Invisibility are staples for a reason, and for our lower levels we unlock perks like Magic Circle Against Good and the very amusing Brilliant Energy Arrow.
Lastly: Minor Creation is online! This is an insanely versatile multitool, giving us powerful drugs and poisons for free, creating floods of oil or acid (get creative) and even combining with Forgery to create any official document we might need after only a few rounds of stalling for time. After all, paper and ink are both vegetable-based; just because most Minor Creation abuse focuses on craft doesn't mean ours has to!
ECL 20
"So that's what those edited records were concealing: a creation forge, in blatant violation of the Treaty, in the depths of Sharn itself? It'd mean life in Dreadhold if anyone heard about this, Merrix, but fortunately for you, I'm more than willing to let you continue your work here. In fact, if you want to work on this so badly, I'll happily take day-to-day management of Cannith South out of your hands..."
The final levels of cerebrex give us some small boosts to CL and spells per day and grant blindsight, finally freeing our human PC from the need to repeatedly cast Ebon Eyes. Eidetic Memory is a great toy here, giving us a sizeable bonus to all the intelligence-based skills (forgery, decipher script, knowledge, craft, spellcraft) that we love to use.
Clairvoyance is more useful than it might seem: we can have up to ten sensors active at the same time, giving us a small network of Magical CCTVs across the city. It's great for tracking the movements of targets, aiding a guard raid by covering exits, and observing new leads to follow up on (or, alternatively, poor sops to blackmail). There's also a bit of a weird level where we have Clairvoyance but not yet Blindsight, which lets us use the 'natural darkness' clause of Clairvoyance to have at least a little bit of sight in a 10 ft. radius area, if all else fails.
Also note this very cool use of Clairvoyance: find a spot within 60 feet of your target, say, in the building on the other side of the street, or several floors up in the same tower, cast Clairvoyance and begin carefully studying the target, cast Sniper's Eye, cast Brilliant Energy Arrow and fire off a Death Attack through the walls with no cover or concealment penalty. If surprise and invisibility somehow isn't enough to let you make sneak attacks (Uncanny Dodge, I guess), use Vital Strike to sneak attack anyway. Because we don't have to spend any Assassin slots on Clairvoyance, we can do this four times per day in four different locations, firing up to 2-3 bolts from all of them.
Twist the Neural Skein, like Aggression Mastery, is easier to use for us thanks to our focus on stealth, and makes us a bit more resistant to the shenanigans casters can pull. Ambushing a cleric with Death Ward? Kidnapping someone who's got a Silent Dimension Door prepared? Unenthusiastic about having your stealth ruined by Glitterdust? Just look right into their brains and jumble up the spell! Interestingly enough, it seems like you can do the spell-busting part as part of another melee touch attack. Consider using, say, these poison rings we're such big fans of: even if your Death Attack fails to kill, it might still rid your target of their go-to riposte.
Neural Glimpse is great if we want our skill-bombs to hit even harder. GotA + NG takes two (or one?) rounds to set up and gives us a +27 bonus to several key skills before items (your uninvested Diplomacy is suddenly enough to turn indifferent people helpful, or turn unfriendly people helpful with some minor item usage). It might even make the affected skills count as 'intelligence-based' and thus a valid target for Eidetic Memory, but that's up to your DM. The AC-boosting mode is yet another tool in our broad arsenal of 'what to do when trouble comes looking for us'.
Other than that... We get our third attack, Martial Stance ups our damage some more; Inscrutable Mind is a bit too passive to highlight but is kind of notable for stopping people from finding our your secrets the brutish way (cast Dominate Person and have you tell them), in addition to shoring up the fear penalty from Craven. The illusion resistance is pretty cool as we don't have True Seeing, Mindsight, or any of the other ways to easily identify more advanced illusions.
Also, we get a dexterity boost, which is mostly pointless but makes us a bit more resistant to Shivering Touch (an unmodified 3d6 roll has a 16% of paralysis before the boost, but only 9% after) and increases the modifier increase we get from Symphony of Nerves from +2 to +3.