HP Casting?

[MaxiDuRaritry]

In Ravenloft: Champions of Darkness there's a "metamagic" (read: not metamagic) feat that allows you to spontaneously swap a spell for another you know by sacrificing hp, but it's terribly worded, to the point that I'm not really sure how it's supposed to work. Are there any other ways to sacrifice hp or ability score damage instead of spell slots/spell points/power points in 3e or Pathfinder?

[ATHATH]

Probably not, because that'd be stupidly broken.

The Blood Magus PrC might be close to what you're looking for, though.

[Segev]

There's Body Fuel (http://www.d20srd.org/srd/psionic/psionicFeats.htm#bodyFuel) for psionics.

[ATHATH]

Actually, on second thought, I think that the Runescarred Berserker PrC might work/fit.

[Coretron03]

The closest I can see is escalation mage which lets you make caster level checks to add pre dertermined free metamagic and if tou fail the caster level check you lose hp. It also gives some bonus hp. Its from faiths of Eberron

There is a pathfinder conversion, although its 3rd party in a update for ebberon and as far as I can tell much more powerful as you can chose the metatmagic. If you took its 4th level ability, sacrificing 15 hp for free metamagic up to a +3 adjustment and used it on echoing spell metamagic you can effectively sacrifice 15 hp for another spell slot. Class is a bit borked look though. Class here https://sites.google.com/site/eberronpathfinder/conversion-info/classes/prestige-classes/escalation-mage

[ShurikVch]

Maho Metamagic of Maho-Tsukai (Oriental Adventures), Blooded Metamagic of Tainted Scholar (Heroes of Horror), and Tainted Metamagic of Tainted Sorcerer (Unearthed Arcana) are all allow to mitigate metamagical "cost" with Con damage

The Ritual of the Dark Flames (Lords of Darkness) allow to cause 1d6/HD of NE damage as a ranget touch attack withing 10'; it's at-will, but cause 2 Wis damage

Book of Blood (Magic of Faerûn) allow to use finger of death 1/day, but every use permanently drains 1 hp
Word of Chaos’s Power (Magic of Faerûn) effect similar to rod of wonder, but every time you use the Word, you take 1 Wis drain

Elder Druid PrC (Dragon #286): capstone ability allow - at the price of few Con damage and (maybe) also aging some years - to regain spent spell(/slot); yes, just like that - elan, Naberius, and never-run-out-of-spells...
Cannibalize Spell feat (Dragon #349) allow to gain power points by sacrificing prepared spell or slot; combine with previous entry to recharge your pp pool

[ATHATH]

If Ability Score damage counts, would the Dark Speech and Dark Whispers feats suit your purposes? How/What about Sanctified and Corrupt spells?

[Venger]

eldritch corruption also seems thematically relevant.

[Slipperychicken]

Blood Money (http://www.d20pfsrd.com/magic/all-spells/b/blood-money/) (PF) lets you take hp and strength damage to replace costly material components. If you have a good setup, you can save yourself a lot of gold on costly spells.

[ShurikVch]

Blood Component CF of Blood Magus or Tainted Scholar PrC allow to replace cheap material components and increase the spell's power; CF of the same name from Maho-Tsukai or Tainted Sorcerer (http://www.d20srd.org/srd/variant/campaigns/taint.htm#taintedSorcerer) PrC allow to replace even costly material components with copious amount of blood

Investigator class from Call of Cthulhu d20 able to cast potentially unlimited amount of spells per day, but, by casting spells, take ability damage, ability drain, and sanity (http://www.d20srd.org/srd/variant/campaigns/sanity.htm) damage

By combining pain Mastery feat, Aeshkrau illumian power sigil, and spontaneous casting, you - in theory - may get some amount of bonus spells (but only for 1 encounter)

[Ruethgar]

Spheres of Power has a casting drawback that causes you to burn health with your spherecasting, it has some limits on healing, but if you can get some form of temp HP it can be pretty awesome. Also taking that drawback lets you use pick the Con for casting trait making you all the more SAD.