Exaros the mech dragon

2{u/b}{u/b}{t} mill the top {x} cards of target opponents library where {x} is equal to Exaros the mech dragons power
{5}{u/b} mill the top four cards of your library. You may play cards with CMC three or less from your graveyard without paying their mana cost
Exaros the mech dragons attack is equal to the amount of instants in your graveyard