D&D4 Wiki
Scorpion strike
rogue attack 23

Requirement: wielding a light blade.
Trigger: An ally damages a creature adjacent to you

Target: the creature your ally damaged

Attack: Dexterity vs. AC

Hit: "2[W] + Dexterity modifier damage."

Miss: "The target makes a melee basic attack as a free action against an adjacent target other than you. You choose the target of its attack."
Brutal Scoundrel: Shift a number of squares equal to your Strength modifier after this attack, whether or not you hit.[PH:125]

Scorpion strike is an encounter power available to rogues at 23th level.