Some kind of demiurge who is alluded to at several points in the game, but only twice specifically.
"Why not ask, Who is the Creator? Or, What is the All? Questions thus asked will remain, in perpetuity, unanswered, for they are in truth unanswerable. To condense all knowledge of the Creator into one answer is futile, as is any attempt to define the All without describing every single element that makes up the All."
"These Lands are blessed by the Creator, shaped by men, yet wild and free and fertile, home to the greatest cities, the most precious sights and the most cultured and civilised people in Arcady."
This mention of both a "Creator" and "the All" as separate concepts implies that the Creator is some kind of deity in the non-absolute sense; a demiurge who made the world presumably ex nihilo, as Urthrin specifies that the land was subsequently "shaped by men". The implication is that the Creator is a god who is possibly eternal, but probably not omnipotent.
In view of the description of the All, however, this characterisation becomes problematic. As is argued in the article on that subject the Creator is most likely a demiurge as in an Emanationist theology.