Devour Essence

You draw upon the very essence of one creature you can see within range. That creature must make a Constitution saving throw, living creatures have disadvantage on this save, or suffer 12d10 necrotic damage, and suffer 1d4-1 levels of exhaustion (minimum 1). You heal yourself for half the damage dealt, and are able to change your creature type to that of the target, stealing any damage and condition vulnerabilities, immunities, and resistances for 1 minute, or until you end the effect as a bonus action, or lose concentration. On a successful save, the target suffers half the damage, and you are still healed for half the damage dealt. 

Higher levels. At higher levels, when cast with a 9th level spell slot, the damage increases to 15d10 necrotic damage, and they suffer 1d6-1 levels of exhaustion.