Essence Siphon

You harvest the life essence from up to three creatures, draining them and bolstering yourself. 

Target up to three creatures within 60 feet of you. These creatures must succeed on a Constitution saving throw against your spell save DC, or suffer 3d6 necrotic damage and have disadvantage on their next attack roll due to the stress. Successful saves take half damage and do not have disadvantage on their next attack roll. You regain half of the damage dealt with this spell as hit points. If you go above your hit point maximum, the remaining healing is transferred into temporary hit points. 

Casting this spell at higher levels increases the damage by 1d6 per spell level.