Xellmarill is a figure of legend, the unstoppable idea taken form.
He attacked heavens & hell band was only stopped by Sigils' Bladed Queen.
He was worshipped by a cult on Easune, most notable Leanus who attempted to kill Shieyth in Xellmarills name.
The Friends of Rainbow found him slumbering in the Death Hole beneath Fredericks Enclave awaiting for when enough blood and heat had been generated for him to awaken.
"Xelmaril Sends His Regards"
The party first heard of Xelmaril when Leanus, the archivist rescued by the Friends of Rainbow earlier, attempted to assassinate Shyieth while saying these words.
What little they have learned since is the stuff of myth and legend, vague but ominous.
"Xelmaril" is reputed to inhabit, or in some sense 'be' the 10th circle of hell, as all evil seeps downward, concentrating and increasing as is goes. It is said it drips down to a lake where it then dissipates. Or it should. But deep in the past the evil concentrated into the birth of an evil Titan - Xelmaril. He caused havoc in the 9 circles, destroying some and then making his way to the outerlands too, even killing some gods. Xelmaril itself wasn't a god or even really a creature, more an idea taken form, that kept growing back while the celestials kept killing it.
Everything seemed unable to stop him. But then he turned towards the spire below the City of Doors, and that was his mistake.
He was destroyed, or put to rest, in some way, by the Bladed Queen.
Forbidden Legends
This book was found by the party in the house of Leanus in Iceport. It contains a number of very obscure stories, including one about Xelmaril. It is very symbolic and metaphorical, but seems to suggest that Xelmaril would return to reset things again, and that there would need to be a great deal of heat and death in some fashion to bring this about.