Nils, Discipline Enforcer is a rare Legendary Creature — Human Cleric card from The List (PLST), collector number C21-20.
At the beginning of your end step, for each player, put a +1/+1 counter on up to one target creature that player controls. Each creature with one or more counters on it can't attack you or planeswalkers you control unless its controller pays {X}, where X is the number of counters on that creature.