Thoughts on creating a Spellcaster that uses hit points for spell casting instead of spell slots.
I was just wondering about home-brewing a walock NPC who's patron saps hitpoints in exchange for spell casting. Essentially, as long as the character has hitpoints they can cast as many spells as they like but with each one it takes a toll.
Balance is obviously an issue here in preventing them from just being healed by the party and used as a spell battery in exchange for healing but Im thinking perhaps balancing that out by making con saves to prevent them from passing out from blood loss or something from wounds appearing on their body as till for the spells. Aswell as that, spells would have different hit point costs that scale with levels like a first level costing 5, 2nd 10 and so on.
Backstory and lore could be pretty interesting as to how they received their patronage, why they took such a harmful deal, they could be good or evil depending on their motives for doing so and other fun stuff!
Let me know what you think or if you have any ideas!
There's something like that in Pathfinder. Not with a warlock, but a separate class named the kineticist. Basically an elementalist who uses CON as his spellcasting stat, and who's attack reduce his max HP. You could take a look at it for balance, but be aware that the class doesn't use normal spells, so it might be tricky.
Otherwise I also know the Blood Mage sub-class in the Pillars of Eternity game, where you have an ability to cause damage to yourself in exchange for spell slots. The amount of damage you cause to yourself equals the level of spell slot your recover. It's powerful, but not as overpowered as one might due to how the system is designed. Basically you use the same stat for single target attacks and for your special ability, meaning that if it's too high you're quickly gonna kill yourself.
I think the max HP reduction is the way to go, so as to avoid balance issues with healing. That's if you want your class to still have limited spells in the same vein as normal spellcasters, just using health instead. I seriously recommend to take a look at Pathfinder's kineticist. It's a very weird class, but I think it may fit what you're looking for.