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No escape
rogue attack 17

Requirement: wielding a crossbow, a light thrown weapon, or a sling.

Trigger: An enemy leaves a square adjacent to you or enters a square where it has cover or concealment against you

Target: the triggering enemy

Attack: Dexterity vs. AC

Hit: "2[W] + Dexterity modifier damage, and you knock the target prone."[MP:83]

No escape is an encounter power available to rogues at 17th level.