A bone devil has been confused by a sorcerer and is currently engaged in combat with a summoner who attacked him. Can the osyluth activate his fear aura (free action) in addition to his full attack or is something in the "confused" or "attacks attacker" states that would prevent it?
That may actually prevent him from activating it. If you rolled anything but "act normally" under the confusion table anyway. I'm not 100% certain that all fear auras require a free action to activate, but I know most of them do.