Tyrigosa returned to the Nexus after her trip to Outland with Nether Dragon eggs. The Nether dragons cured Malygos of his madness and he came to his senses to see that the world was recklessly using magic. He feared that this reckless use of magic would lead to the Burning Legion returning to Azeroth. Thus he waged war on the Kirin Tor.
Malygos is involved in many quests in the Borean Tundra. In what questline his consort is killed by the red dragon Keristrasza, in anger he captures Keristrasza and forces her to become his consort. He then locks her within the Nexus.
In World of Warcraft: Wrath of the Lich King. Malygos is a new Onyxia style raid boss which can be found in the Eye of Eternity, one of the 10 - 25 man raids in Wrath of the Lich King. In the battle, Alexstrasza and the Red Dragonflight assist you through parts of it.