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Unbalancing attack
rogue attack 13

Requirement: wielding a light blade

Target: one creature

Attack: Dexterity vs. AC

Hit: "3[W] + Dexterity modifier damage, and the target cannot shift until the end of your next turn. If the target provokes an opportunity attack from you before the start of your next turn, you gain a bonus to the attack roll and damage roll with the opportunity attack equal to your Strength modifier, and you knock the target prone on a hit."[PH:123]

Unbalancing attack is an encounter power available to rogues at 13th level.