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.
For each player, choosing a target creature that player controls is optional.
16 Apr 2021
All counters on a creature are considered when determining the value of X, not only +1/+1 counters.
16 Apr 2021
Your opponents can choose not to pay to attack with a creature with one or more counters on it that attacks "if able." If there's no other player or planeswalker to attack, the creature simply doesn't attack.