Devastating Blow is an encounter power available to the Barbarian class at level 17.[PH2:57]
Devastating Blow
Barbarian Attack 17
Your powerful blow shatters your target's defenses.
Melee weapon
Target: one creature
Attack: Strength vs AC
Hit: 4[W] + Strength modifier damage, and the target takes a -2 penalty to AC until the end of your next turn.
Thaneborn Triumph: The penalty to AC equals your Charisma modifier.