Abilities triggering at low HP

[Beni-Kujaku]

The last iteration of Villainous Comp saw a villain with Berserker's Strength and the epic destiny allowing to continue fighting down to negative HP equal to half your max. Positive HP. It makes for an interesting two-phase boss‚ but can we do better?

What are some abilities that trigger when you're at low HP (ideally less than half) that could be used for a multiple-phase boss?
(I'm not talking about Contingency spells that you could set to trigger when you're at low HP‚ but abilities that are inherently tied to you being already beat up)

[Saintheart]

Most obviously, Diehard. You're made automatically stable if you hit -1 through to -9 hitpoints, and you can act as though disabled, not dying.

Test of Mettle for the Knight does something similar.

[Inevitability]

Healing Devotion automatically triggers when you're below 0 HP, which would work kinda well with the aforementioned epic destiny trick.

[Biggus]

Fortunate Fate (SpC) auto-heals you if killed by HP damage or any other effect Heal works on.

(Greater) Healing Armor (MIC) auto-heals you if you fall to -1 to -9HPs.

Tabard of Valor (CCh) gives you mettle if your HPs fall to half your normal maximum.



Test of Mettle for the Knight does something similar.

Do you mean Loyal Beyond Death?

[flappeercraft]

Pact of return somewhat works like that? It’s more on death trigger than HP though.

If I’m not mistaken there was also a barbarian acf that made it rage when you were low in HP.

[Beni-Kujaku]

Ok, thank you everybody! It seems, except Tabbard of Valor and Berserker's Strength (flapeercraft, that's the name of the barbarian acf), most such effects occur when you're below 0 HP or should straight out be dead. Unfortunate.



Pact of return somewhat works like that? It’s more on death trigger than HP though.
"I expect to die when my Contingent [when I reach -10 HP] Delay Death expires"

[ciopo]

Final strike technically qualifies? After all, it goes out of it's way to quantify the triggering event to be dropping to -10 or lower, and not actual death. Should therefore work with blade of ragnarok

[ViperMagnum357]

There is Frenzied Berserker and deathless frenzy-while you have severe action restrictions, you cannot be killed by hit point damage until it ends, and if you have a way to get healing in spite of the action restrictions, you could survive. It could essentially add a third phase to such a fight: high health (50% and up), low health (50% to -50% via Blade of Ragnarok), and desperate, 'boss turns red, changes tactics and starts coming apart at the seams in a last desperate attempt to kill the heroes' (-50% or less until the frenzy ends): force the heroes to run out the clock in survival mode, or until they can connect with an effect to defeat the boss without hit point damage.

[Telonius]

It's a magic item, but the Rapier of Desperate Measures (MiC) becomes Keen when you have fewer than full hit points, and gets the Speed property if you have less than half. (It's a Relic of Olidammara, so you need to either have the True Believer feat or sacrifice a spell slot).

[Biggus]

Stalwart Pact and Renewal Pact (both SpC) trigger when you're on half HPs and when you're subjected to one of a list conditions respectively.

Edit: there's also the Outcast Champion class (RoD) which gains a rage-like ability when it reaches half HPs.

Edit 2: and Boots of Desperation (MIC) which activate at half HPs. Only lasts one round but they're fairly cheap.

[Jervis]

Iirc theres a Barbarian variant from dragon mag that rages constantly when their hp is below twice class level or something

[ShurikVch]

Monsters:
Badger (https://www.d20srd.org/srd/monsters/badger.htm) - Rage: on any hp damage
Wolverine (https://www.d20srd.org/srd/monsters/wolverine.htm) - Rage: on any hp damage
Dire Badger (https://www.d20srd.org/srd/monsters/direBadger.htm) - Rage: on any hp damage
Dire Wolverine (https://www.d20srd.org/srd/monsters/direWolverine.htm) - Rage: on any hp damage
Guardian Familiar (Dangerous Denizens - Kingdoms of Kalamar) - Nine Lives: up to 8 times killed Guardian Familiar instantly comes back to live, and each such revival gives it +1 attack/damage/AC/SR/HD and +5' speed; at the start Guardian Familiar looks like a small black cat, but with HD gained it grows up to size of a bobcat
Templates:
Bloodrager (Bestiary Of Krynn, Revised) - Rage (unlike the aforementioned Animals - not unlimited in duration)
Kaiju (Dragon #289) - Battle Frenzy: when at 25% hp (or lower), gets +4 Str and additional attack with primary natural weapon (-5 on attack)
Savage Creature (Dungeon #140) - Death Throes: when Savage Creature is killed, it makes one last bite attack (despite it should be already dead at this point), after which - melts into a puddle of acid
Feats:
Arcane Toughness (Player's Handbook II) - if at 0 hp (or less), as immediate action consume a spell (or spell slot) - heal amount of hp equal to the spell's (or slot's) level
Boar's Ferocity (Complete Divine) - if at 0 hp (or less), spend one use of Wild Shape as a free action (even if it isn't your turn) to act for a minute as if not disabled or dying
Bonded Familiar (Player's Handbook II) - 1/day, if attack or effect should put you below 0 hp, Familiar able to take damage instead of you
Chant of Fortitude (Complete Adventurer) - Bardic Music gives benefits of Diehard feat for 1 turn
Death Frenzy (Lords of Madness) - it's feat for Aboleths; when number of its hp is less than number of its HD, Aboleth gets +4 morale on Str and Dex, one additional attack each round, +10' land speed, +20' swim speed, fast healing 2, and the threat range for all its natural attacks doubles. Effects lasts until the Aboleth either healed or dead. At the end, Aboleth take 1d4 Str damage
Faith Unswerving (Tome of Battle) - One Last Strike: if reduced to 0 hp or less, as immediate action move 5' and make a single melee attack with a +5 bonus and damage bonus = character level
Fearless Destiny (Races of Destiny) - 1/day, effect which should reducing you to -10 hp or less, instead leaves you at -9 hp and stable; effects which killing without reducing hp still work as usual
Involuntary Rage (Savage Species) - suffer 50+ damage in one attack, make your Fort save, get +4 Str&Con, -2 AC; effect lasts until the end of the combat (then - fatigue)
Pain Mastery (Savage Species) - +2 Str for every 50 damage until the end of the encounter (then - exhaustion)
Pious Defense (Complete Divine) - if would be reduced to 0 hp (or less), can spend 1 faith point to take only half of damage
Prophecy's Mind (Magic of Eberron) - any time you would be reduced to -10 hp (or lower), expend your psionic focus to instead be reduced to 0 hp; doesn't work on effects which kill without hp damage
Remain Conscious (Masters of the Wild)
Ritual Blood Bonds (Player's Handbook II) - if you see another participant drop to 0 or fewer hp due to an opponent's actions, you get +2 morale on attack against that foe
Scramble (Savage Species) - like Defensive Roll (see Rogue), but only against blows (not spells or SA), and negates damage completely
Shifter Ferocity (Races of Eberron) - at 0 hp, don't lose hp for strenuous actions, don't fall unconscious from -1 to -9 hp, and don't lose hp automatically each round when at -1 to -9
Slayer of Dragons (Dragon Magic) - Stand Alone: at anytime during the next 24 hours, any participant is reduced to -1 or fewer hit points by the attack, breath weapon, spell, or other ability of a dragon, all other participants gain a +2 morale bonus on attack rolls and damage rolls against dragons, as well as on saving throws against the attacks of dragons for the next 10 rounds or until the wounded character is restored to 0 or more hit points
Tempting Fate (Complete Scoundrel) - 1/day, when at positive hp and would be dealt enough damage to kill you, expend one luck reroll as an immediate action to take only enough damage to reduce you to -9 hp; you automatically stabilize
Wedded to the Light (Dragon #358) - at 13th level, Familiar is able to revive its master at the cost of self-sacrifice: master comes to life with -1 hp, stable, and even without the level loss (but lose 200 XP per level for dead Familiar)
Classes:
Factotum 13 - Cunning Dodge: 1/day spend 4 inspiration points as an immediate action to ignore the damage that would reduce you to 0 or fewer hp
Rogue 10 - Defensive Roll (https://www.d20srd.org/srd/classes/rogue.htm#specialAbilities): 1/day Ref save for half-damage vs whatever which would reduce Rogue to 0 hp (or less)
Spirit Shaman 19 - Favored of the Spirits: Heal effect in case Spirit Shaman is reduced to 0 hp or 0 in any of ability scores; in case damage reduce hp to -10 or lower - Heal works anyway (i. e. Spirit Shaman isn't dead); it costs 1000 XP
PrC:
Blood Magus 1 (Complete Arcane) - Stanch: automatically stable below the 0 hp
Cavelord 10 (Underdark) - Bones of the Earth: if damage lowers hp to -10 or less (or if unstaunched bleeding would cause the cavelord's hit points to drop to -10 or below), Cavelord can make Fort save, and if successful - turns into stone (as the spell (https://www.d20srd.org/srd/spells/fleshToStone.htm)). The stone form is immobile and looks like a part of natural surrounding. Cavelord reverts back 24 hours later - with 1 hp and no prepared spells
Daidoji Bodyguard 8 (Oriental Adventures) - Defensive Roll (see Rogue)
Death Delver 10 (Heroes of Horror) - Nine Lives: up to 9 times, when Death Delver should die from whatever attack or effect, they are drop to -9 hp and stable
Death's Chosen 3 (Libris Mortis) - Bravery of the Chosen: when their master at 50% hp (or less), Chosen got +2 morale bonus on melee attack and damage rolls//it isn't impressive - but still there
Divine Seeker 4 (Player's Guide to Faerûn) - Divine Perseverance: 1/day, when at -1 hp (or lower), heal 3d6 + Cha bonus (if any) hp
Eye of Gruumsh 10 (Complete Warrior) - Sight of Gruumsh: does not go unconscious when reduced to negative hp
Fiendbinder 5 (Tome of Magic) - Bind Tormented Soul: 1/day for 3 rounds can act normally while disable or dying; doesn't die at at -10 hp, but reduced to a single Move or Standard action (thus, no full-round actions possible; standard actions inflict the usual 1 hp damage, move actions are "free")
Frostrager 1 (Frostburn) - Freezing Blood: automatically stabilizes at negative hp (but still takes 1 hp damage for actions)
Geomancer 7 (Complete Divine) - Rage (see Wolverine)
Giant-Killer 4-10 (Silver Marches) - Diehard: at 4th level remain conscious at -1 to -9 hp, but actions hindered; at 8th level - remain conscious and acts as usual; at 10th - can make Con checks to not die at -10 hp or lower (would still die at -30)
Glaive of Azharadian 2-9 (Dragon #293) - Unvanquished Even in Death: 1/day, when down to from 0 to -9 hp, gets Con bonus (+2 at 2nd level, +4 - at 6th, and +6 - at 9th); still dead at -10 hp; lasts for 1 class level per round; don't activates on self-inflicted damage, or wounds from allies
Gnome Giant-Slayer 10 (Complete Warrior) - Defensive Roll (see Rogue)
Illumine Soul 4 (Complete Psionic) - Positive Energy Healing: once in 24 hours, if down to 0 hp (or less), instantly heals 5d8+5 points of damage (enough to prevent death - if hp got to -9 or more)
Outlaw of the Crimson Road 10 (Song and Silence) - Cheat Death: mostly like Defensive Roll (see Rogue), but allow use Improved Evasion to completely negate the damage (only in the case if the damage source allows Ref saves)
Outcast Champion 3 (Races of Destiny) - Desperate Fury: when at less than 50% hp, fatigued, or exhausted, get +2 morale on Str/Dex/Con for 3 rounds (+ Con bonus); effect ends if Champion become frightened, panicked, or cowering
Sanctified One - of Wee Jas - 3 (Complete Champion) - Sanctified Fall: if at -1 hp (or less; or even - killed outright), as immediate action can either make single attack or cast a spell; +class level to either attack roll or caster level
Shadowdancer (https://www.d20srd.org/srd/prestigeClasses/shadowdancer.htm) 5 - Defensive Roll (see Rogue)
Tainted Scholar (Heroes of Horror) - Stanch (see Blood Magus)
Telflammar Shadowlord 5 (Unapproachable East) - Shadow Discorporation: at any damage that put them below 0 hp, Shadowlord can try Ref save; if successful - they vanish and appear on the next sunset (in the place of their choice, but within 1 mile from the point of discorporation)
Warmaster 9 (Sword and Fist) - Die for Your Country: any allies within 30 feet of you can continue to fight while disabled or dying without penalty
Weretouched Master 3 (Eberron Campaign Setting) - Rage (see Wolverine)
Yakuza 1 (Oriental Adventures) - Defensive Roll (see Rogue)

[loky1109]

AD&D 2 Scorpion has 2 sting attack (instead 2 claws and 1 sting) when in 1 hp.
It's very distant, but...

[Biggus]

<loads of stuff>

Woah...

Serious question: how the heck do you find all this? I spent a while thinking and looking and I only found a fraction of what you did.

[loky1109]

Woah...

Serious question: how the heck do you find all this? I spent a while thinking and looking and I only found a fraction of what you did.

I think he's just very smart. )

[Maat Mons]

Any effect that triggers when Raging can be triggered by low HP, through Berserker Strength. Dragon Shaman’s Vigor aura triggers at half HP.

[ShurikVch]

Any effect that triggers when Raging can be triggered by low HP, through Berserker Strength. Dragon Shaman’s Vigor aura triggers at half HP.
Then Half-Vampire template too (Libris Mortis) - its Fast Healing works only when creature have less than half of their full hp

[Gorthawar]

Didn't see it mentioned yet. The rubicund frenzy armor crystal gives +1/+3/+5 morale bonus on weapon damage and saves vs. fear when below half health. (MIC pg26)

[ShurikVch]

Tabard of Valor (CCh) gives you mettle if your HPs fall to half your normal maximum.
Then also Crest of Valor for +2 on damage rolls, and Periapt of Valor for +2 AC

Races of Faerûn have Mask of the Implacable: it gives +4 on attack and damage rolls vs whoever damaged you for more than 50% of your health - as long as you don't attack anybody else