Volhm's Energy Drain

You raise your hand, sending inky tendrils of darkness towards your target. On a successful Constitution saving throw, taking 10d12 necrotic damage on a failed save, or half as much on a successful one. If the save fails, the target also has its hit point maximum reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this spell reduces its hit point maximum to 0. 

In addition, you gain temporary hit points equal to the damage dealt. This spell does not affect undead or constructs.