Reaper's Challenge is a 15th level swordmage Daily power that was introduced in the 372nd edition of Dragon (magazine).
Reaper's Challenge
Swordmage Attack 15
You intercede for an ally, enshrouding your foe in a specter of death that can only be exorcised by facing you.
Close burst 5
Trigger: A creature attacks an ally
Target: Triggering creature
Attack: Intelligence vs. Will
Hit: 2d6 + Intelligence modifier necrotic damage, and the target is weakened until the end of your next turn.
Effect: Until the end of the encounter, if the target makes an attack that does not include you as a target, the target is weakened until the end of your next turn.