Defender aura is a at-will power granted to Berserkers as a class features at 1st level.[HotF:44]
Defender aura (berserker)
Berserker Utility
Personal
Effect: You activate an aura 1 that lasts until you end it as a minor action or until you fall unconscious. While in the aura, any enemy takes a −2 penalty to attack rolls when it makes an attack that does not include among its targets either you or an ally of yours who has this aura active. Marked enemies are not subject to this aura.