The Nameless God is believed to be a giant, tentacle monster who controls the Under-Realm. Of course, being in the Under-Realm, nobody really knows if it really exist or not. It hold every person, every soul, with one of its endless tentacles, until the fate of that soul becomes clear. Despite never being seen by anyone for obvious reasons, almost all races in Aravorn believe in the Nameless God and the Under-Realm.
The Nameless God doesn't really have worshippers, but some races, like the snow elves, believe the Kraken to be a manifestation of this God, so they fear but at same time respect him.