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Ferret out frailty
rogue utility 6

Prerequisite: Insight trained

Effect: "Until the beginning of your next turn, you gain combat advantage against one target within line of sight. If the target is taking the attack penalty from one of your rattling attacks, you instead gain combat advantage until the end of your next turn."[MP:77]

Ferret out frailty is an encounter power available to rogues at 6th level.