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Dreadful word
warlock (star) attack 1

Target: one creature

Attack: Charisma vs. Will

Hit: "2d8 + Charisma modifier psychic damage, and the target takes a −1 penalty to Will defense until the end of your next turn."
Star Pact: "The penalty to Will defense is equal to 1 + your Intelligence modifier."[PH:132][U :W]

Dreadful word is an encounter power available to warlocks at 1st level.