Xyius was an Elder Being who previously ruled over the seas and waterways of Oerth, and father of Sionnaine thanks to a bargain Leannan struck with him to stop him flooding the entire Oerth following the first death of Marduk. His power was later absorbed by the powerful Kobold sorceress Bel Kuko, by mechanisms unaware even to Bel herself.
Forms: Xyius was the embodiment of the concept of agnosiophobia (the fear of the unknown), and so took the form of a colossal sea monster, with hundreds of tentacles, the head of an anglerfish, and a body like a massive sea serpent.
Death: Xyius was killed by Sionnaine at some point either at the end of the Second Age or the beginning of the Third Age, and Sionnaine took his place as the ruler of the oceans. According to visions experienced by Bel, his body is apparently somewhere in the endless seas of Neró, presumably dumped there by Sionnaine following his death.