Dead Magic

You fuel your magic with your own life force, gaining increased spellcasting but wracking your body in the process. 

For the duration or until you choose to end the spell as a bonus action, you lose hit points instead of spell slots whenever you cast a spell of 1st level or higher, and you may choose to use Constitution as your spellcasting ability. The number of hit points lost is equal to three times the level of the spell cast, and you may not cast a spell in this way at a greater level than you already possess spell slots for. This damage ignores temporary hit points.

When you lose hit points in this way, your hit point maximum is reduced by the same amount until the spell ends. If your hit point maximum is reduced to 0 in this way, you die.