Come on- that has to be a dragon tail!
Chaos dragon?!
(Maybe he was kicked out of the sky castle and reborn- and that’s why he was so Op from birth…. And the sky castle doesn’t want him there cos he was the prior ruler).
That or he was never destined for the sky castle and the underground is a new platform/dimension that no one knows about yet- maybe born from all the fallen players wishes and hopes?