Thrill-Kill Assassin | Magic: The Gathering
Formats Thrill-Kill Assassin is Legal in
Official Oracle Text for Thrill-Kill Assassin
Unleash (You may have this creature enter the battlefield with a +1/+1 counter on it. It can't block as long as it has a +1/+1 counter on it.)
Official Rulings for Thrill-Kill Assassin
4/15/2013 : The unleash ability applies no matter where the creature is entering the battlefield from.
4/15/2013 : A creature with unleash can't block if it has any +1/+1 counter on it, not just one put on it by the unleash ability.
4/15/2013 : Putting a +1/+1 counter on a creature with unleash that's already blocking won't remove it from combat. It will continue to block.