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.
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.