Roving Around Athas
Lord of the Shadowsea
Dagon, colloquially known as Bane, is an ancient and powerful Demon Lord who was trapped on Athas in a green little orb in an isolated and unguarded monastery in the middle of nowhere that the party was able to unleash upon the universe again.
Maybe that isn’t quite right, but the PCs aren’t sure of any other possible explanation. They know this being gave them some pretty immense power after they released him. They also speculate his is responsible for invading the mirror world of Sathir with legions of demons.
He appears to have the motifs of tentacles, a green unblinking eye, and water.