Brotherhood Assassin

Shadow, Dash X
Brotherhood Assassin's power and toughness are equal to X. When Brotherhood Assassin enters the battlefield, put a +0/+1 counter on it.
Hunted Y: When Brotherhood Assassin enters the battlefield, you may choose target creature opponent controls and pay Y, where Y is the creature's converted mana cost. That creature gains Shadow whenever it attacks until Brotherhood Assassin is removed from the battlefield.