Thrill-Kill Assassin
Deathtouch 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.)
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Official rulings (4) · Scryfall
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.
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.
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.
The unleash ability applies no matter where the creature is entering the battlefield from.