Arth and Vandun are partners in crime. Literal crime. And villainy. General evil, you know the sort.
Arth and Vandun conspired to try and outright destroy the world. Using the orc legendary gemstone, which provides immunity to all harm, Arth descended to the core of Ilisara to try to destroy the core. Arth allowed himself to be a conduit for Vandun's power, but the legendary gemstone shattered under the intense heat, pressure, and radiation of Ilisara's core. Arth's body was destroyed, and his spirit was evacuated by Vandun. The resulting explosion from the orc legendary gemstone resulted in the eruption of Isandor.
Arth’s ultimate goal is to rebuild his body. He has turned to his worshippers to make offerings to him. When a sufficiently powerful vessel is offered, Vandun will replace Arth’s spirit into the vessel and Arth will reward him by offering his followers to Vandun.