Exava, Rakdos Blood Witch

{2}{B}{R}Legendary Creature — Human Cleric3/3

First strike, haste

Unleash (You may have this creature enter 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.

First strike. This creature deals combat damage before creatures without first strike. · CR 702.7

Haste. This creature can attack and use tap abilities as soon as it comes under your control. · CR 702.10

The unleash ability applies no matter where the creature is entering the battlefield from. (2013-04-15)
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. (2013-04-15)
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. (2013-04-15)
You make the choice to have the creature with unleash enter the battlefield with a +1/+1 counter or not as it’s entering the battlefield. At that point, it’s too late for a player to respond to the creature spell by trying to counter it, for example. (2013-04-15)