Desoloth is the great elemental dragon sung about in children's songs, Desoloth has been sealed for 800 years by the four Prime Dragons Moganth, Udaroth, Cellot, and Zellare. X'Dir opened the 11-11-11 portal for him so he could find someone to free him before he was sealed away forever. In reality, it's all according to Desoloth's plan to become free from confinement so he can devour the elements.
After the Hero was tricked into unleashing him upon Lore, Desoloth went on a rampage in the Etherstorm Wastes, disrupting the balance of the elements and killing many people. He turned As'iiur, one of the four Elemental Dragon Priests, to his side with a promise to spare the dragons under his care, but when As'iiur granted the Hero the power of Air in order to fight him, Desoloth killed him without mercy. His minion X'Dir also captured the famous dragonslayer known as Galanoth to be sacrificed to Desoloth, but when Galanoth tried to kill Desoloth, the great dragon bit off his right arm, which would have killed him if not for the intervention of his friend Warlic, who used all his magical power to save his life, but could not restore his arm.
When the Hero, having rescued the four dragons who would become the Prime Dragons, confronted the great dragon, Desoloth struck down Omom, the cute little lemur who was the pet of Ang'st, the new Priest of Air, for daring to stand against him. Though the Dragon Priests intended to seal Desoloth away in the Dragonplane and bring him back to a more harmonious state of mind, the Hero chose to kill the great dragon to avenge the fallen and end his threat to Lore forever, an act which angered the Dragon Priests and led Ang'st to strip the Power of Air from them.
Unfortunately, when Desoloth's bones were sealed away in the Dragonplane, Drakath showed up and used his Chaos power to bring Desoloth back as a powerful Dracolich, giving rise to Desolich. Desolich would resurface later, corrupting the Dragonplane and unleashing a plague against the dragons of Etherstorm with the intent of transforming them into powerful dracoliches under his control. The Hero stopped this plague at its source, cleansing the Dragonplane and destroying the avatar of Desolich, severing his connection to Lore.