Feat: Blood Magic

Prerequisite: the ability to cast 1st-level or higher spells

You've learned to use your own life force to power your spells. 

When you use your action to cast a spell you may use a bonus action to sacrifice hit points and create a spell slot of 5th-level or lower. If you use your life force to power the spell you immediately take 1d4 necrotic damage per spell level of the spell slot created, and your hit point maximum is reduced by the same amount. You cannot apply resistance or immunity to this damage, and it bypasses temporary hit points or other defenses.

You cannot heal damage dealt by this feat or eliminate your hit point maximum reduction except by finishing a long rest.