Accursed Healer 

Prerequisites: The ability to cast spells or have an ability that restore hit points

You can corrupt your healing spells and abilities with necrotic energy. You gain the following benefits:

When you causes a creature to regain hit points, you can choose to use an accursed version of that spell or ability. The target must make a Constitution Saving throw against your spell or ability save DC, taking nectrotic damage instead of the healing that it would have received on a failed save, or half as much on a successful one. 

When you use a spell or ability that causes a creature to regain hit points that cannot target or affect undead, it can target or affect undead. You may make this choice individually for each creature targeted.

Your healing spells restore a number of additional hit points equal to your spellcasting ability modifier when used on undead.