Opportunity actions let you take an action in response to an enemy letting down its guard. The one opportunity action that all combatants can use is an opportunity attack. Most combatants have no additional powers that use opportunity actions, and those powers that do tend to resemble enhanced opportunity attacks.
Not On Your Turn: You can’t take opportunity actions during your own turn.
Once Per Combatant’s Turn: You can take one, and only one, opportunity action on each other combatant’s turn.
Interrupts Action: An opportunity action interrupts the action that triggered it.
Resolution: It is possible that an opportunity action can itself trigger one or more other triggered actions. When this happens, refer to the rules for Multiple Trigger Resolution.
Opportunity Action Triggers
All opportunity actions use the same trigger conditions. An opportunity action is triggered in any of the 3 following ways:
1. An opponent moves out of a square you threaten. This doesn’t trigger if the opponent shifts instead of moving. An opponent can avoid triggering an opportunity action by using the Withdraw action, which allows them to treat the first square they leave as though it weren’t threatened.