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Collapsing riposte
rogue attack 23

Requirement: wielding a light blade.
Trigger: An enemy misses you with a melee attack

Target: the triggering enemy

Attack: Dexterity vs. AC

Hit: "3[W] + Dexterity modifier damage, and you knock the target prone."
Artful Dodger: The target is also dazed until the end of your next turn.[MP:84]

Collapsing riposte is an encounter power available to rogues at 23th level.