Nightmare Leech

Skulk, haunt (When this creature dies, exile it
haunting target creature.)
When Nightmare Leech enters the battlefield or
the creature it haunts dies, the next instant or
sorcery spell you cast this turn gains cipher.
(After you resolve that spell, then you may exile that
card encoded on a creature you control. Whenever that creature deals combat damage to a player, its
controller may cast a copy of this card without paying
its mana cost.)