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Stunning assist
rogue attack 27

Requirement: wielding a crossbow, a light blade, or a sling

Target: one creature

Attack: Dexterity vs. AC

Hit: "3[W] + Dexterity modifier damage, and the target is dazed until the end of your next turn. If the target is marked by any of your allies, it is stunned instead of dazed."[MP:87]

Stunning assist is an encounter power available to rogues at 27th level.