Persistent menace
rogue attack 25
Melee weapon
Requirement: wielding a light blade.
Target: one creature
Attack: Dexterity vs. AC
Hit: "2[W] + Dexterity modifier damage, and ongoing 20 damage (save ends)."
Miss: "Half damage, and ongoing 10 damage (save ends)."
Effect: Until the end of the encounter, whenever you deal damage to the target, it takes a -2 penalty to saving throws until the end of your next turn.[MP2:70]
Persistent menace is a daily power available to rogues at 25th level.