Warlock that casts spells using hitpoints instead of spell slots

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!