Exid, Shadow Demon

When Exid enters the battlefield, exile the Celestus stack, then enchant target creature you control.
Whenever enchanted creature attacks, so does Exid(Exid cannot attack unless enchanted creature attacks), and whenever enchanted creature attacks or has an activated ability, put a -1/-1 counter on enchanted creature and a +1/+1 counter on Exid.
If enchanted creature goes to the graveyard, enchant another target creature you control.