Levistus

Levistus is an Overlord imprisoned in the Frostfell. Among the Lords of Dust, a group of rakshasa and fiends who seek to free the Overlords and bring about a second Age of Demons, Levistus' greatest servant was the wizard Galather. Galather worked for centuries to free the arch-fiend and, in the year YK 999, he nearly succeeded. Although Levistus never fully physically manifested in Eberron, he was still able to subvert the wills of tens of thousands of dragonmarked individuals, turn the greatest city in the world into a frozen ruin, and nearly destroy the social and economic fabric of Khorvaire. 
Levistus was stopped by a group known as the Troublemakers, but the damage done was incalculable.