Isemay, the Decapitator

At the beginning of your end step, reveal the top
card of your library. If it is a Blue card, target
player puts the top two cards of their library into
their graveyard. If it is a Black card, put a bounty
counter on a creature an opponent controls.
Then put the revealed card at the bottom of your library.
Whenever you cast an Assassin spell, you may
destroy target creature with a bounty counter on
it.