After resolving Dorghar's normal Attacks, you may pick one of the following effects if at least one model was slain by those attacks:
Filth-spewer: Dorgar inflicts D3 Wounds which Ignores Armour Saves on one enemy unit in base contact.
Skull-gorger: If any of the slain models were Characters, Dorghar's Khornate head devours their skulls and Archaon heals D3 Wounds up to his starting value.
Spell-eater: If any of the slain models where Wizards, Dorghar's Tzeentchian head devours them, learning any spells they knew and passing them on to Archaon.