Grevalt, Bloodveil Assassin

Whenever Grevalt, Bloodveil Assassin attacks,
you may pay {4}. if you do, you may put a
Vampire creature card from your hand onto the battlefield tapped and attacking. Return it to
your hand at the beginning of your endstep.
(Even if it is a tribal enchantment subtyped
"Vampire", it still has to be a creature.)
"You shall be treated as I was before I joined the
Veil, foolish heretic."