The Cataclysm

From Guild Wars Wiki

The Cataclysm was a destructive force caused by a forbidden spell that destroyed the Kingdom of Orr and sunk it under the sea. The person who cast this forbidden spell was the King of Orr's advisor, Vizier Khilbron, shortly after the Searing of Ascalon to defend Orr from invading Charr. It was this spell that turned Vizier Khilbron into the Lich Lord.

It is indicated during the latter stages of the Nightfall campaign that the fallen god Abaddon is the source of the destructive power that caused The Cataclysm. Abaddon was bent on revenge against the Five Gods for his ouster from their ranks, so he orchestrated the Charr invasion through his Titans, whom the Charr worshiped as gods, as part of an elaborate scheme to destroy the holy city of Arah, where the Tyrian gods dwelled before The Exodus.