Exava, Rakdos Blood Witch | Magic: The Gathering
Formats Exava, Rakdos Blood Witch is Legal in
Official Oracle Text for Exava, Rakdos Blood Witch
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.)
Each other creature you control with a +1/+1 counter on it has haste.
Official Rulings for Exava, Rakdos Blood Witch
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.