I can't reply to the "Random question - Tu'narath" post, the "Can't reply to a post" post, or the "Thread for collecting changes in 5e from established Planescape lore" post.
Replies may be globally broken? I haven't seen ANYONE reply to a thread for quite a while now :(
Edit: Looks like I CAN edit this post.
In case this doesn't get corrected soon, my intended reply is:
Huh, that's a really good question. But as far as I know, it was never elaborated upon. The Guide to the Astral just says "It's build upon the body of an unnamed god, said to have been aeons-dead when even the githyanki came to the plane."
If I were to invent something for a home campaign, I would probably tie the god to the illithid eimpire in some way. Either as the remains of a god slain by the illithids, or as a remnant of some other empire from even deeper in the past than the illithids.