Anathema of the Undead

When you are wearing this necklace and cast a spell that restores hit points to a creature, you can use your bonus action to deal the same amount of damage (radiant) to one undead creature that you can see. The undead creature must make a WIS save with the same DC as the spell cast or take the full damage (half damage on a success). You can target an additional undead creature that you can see if you decide to take the healed hit points as damage. You can repeat this process indefinitely as long as you don't fall unconscious.