Draining Aura

You pull the Life essence out of each creature within 15 feet of you.

At the end of your turn, each creature within 15 feet of you must make a constitution saving throw.  On a failed save, the creature takes 2d4 necrotic damage. On a successful save, the creature takes half as much.  You regain hit points equal to half the damage taken this way. 

At Higher Levels. When you cast this spell at 4th level or higher, the damage increases 2d4 for each slot level above 3rd

As an additional cost to cast this spell, expend 1 hit die and lose a number of hit points equal to the number rolled.