Twilight Gengar

Shadow Pain — When you cast Twilight Gengar
from your hand, you may have it deal X damage
to any target. If you do, Twilight Gengar deals
that much damage to you.
Whenever Twilight Gengar deals combat damage
to a player, that player may tap a creature they control, it doesn't untap during its controller's
next untap step. If they don't, that player gets a
poison counter.