Devastator

By expending a number of spell slots and paying half your current hit points, you can choose a number of creatures that you can see equal to the number of spell slots used. Those creatures instantly combust in black flames, these flames cannot be put out unless you or the creatures die. Each creature affected must make a Constitution saving throw. On a failed save the creatures take damage equal to their maximum hit points -1, the damage is halved on a successful save.

This spell can only be used once per long rest, however it requires two long rests to recover any spell slots expended using this spell.

At Higher Levels: When you cast this spell using a spell slot of 2nd level or higher, the Difficulty Class for the Constitution saved is raised by two or higher depending on the level of the spell slot used.