Decimation

You send forth a chain of deathly energy into the soul of a creature you can see. That creature must make a Constitution saving throw. On a failure, the soul of the creature is bound to you for ten rounds, and during each of your turns for the duration, you may spend 1 action to deal Necrotic damage equal to 1/10th of the creature's hitpoint maximum, rounded up to the nearest whole number. If the creature would die before the spell ends, you gain temporary hitpoints equal to the amount of damage dealt to the creature using this spell. On a successful save, the creature is dealt Necrotic damage equal to 1/10th of their current hitpoints, rounded up to the nearest whole number, and the spell ends. The damage dealt by this spell ignores all damage resistance, but does not ignore  damage immunity.

At Higher Levels: When you cast this spell using a spell slot of 6th level or higher, you may target 1 additional creature for each slot level above 5th.