Like the Head of Onyxia, Nefarian's head may be delivered either to King Varian Wrynn in Stormwind for the Alliance or Thrall in Orgrimmar for the Horde. Wrynn will send you to Field Marshal Afrasiabi in the Valley of Heroes, while Thrall will send you to Overlord Runthak near the gates of Orgrimmar. Turning in the head grants everyone in the city the Rallying Cry of the Dragonslayer.