Caius

Physical Description
Caius Stands around 18 feet tall (5.4 Meters), he has a pair of long horns that curve his back, with a second pair of smaller horns directly below the first pair. He has applied burnmarks in evershifting shapes cascading down his limbs, and a large ruby at the center of his chest. His eyes are white, and he carries a large greatsword, usually in one hand.

Research on Caius
Research on the Demon lord has been difficult but searching through tomes of the ancient world has shown results. The following notes are things you have discovered about Caius.

Caius is a local name from one of the former continents, thousands of years prior. The demon himself is known by many names but pre-fall throughout the plane he was known as Areth-Glithavano, the King of Hell. The position had to be earned, however during the fall it was lost as what was happening within Heaven and Hell. While day to day knowledge of the outer realms is not common on the material plane, but major events are and after the fall Caius disappeared from history.

From previous description of him he was known to be rather proficient at manipulating souls through what is known as The Red Mist. Absorbing the power of other Devils and Outsiders was the way Caius got to his position as king, yet the descriptions of him show him as much more powerful and terrifying than what you saw.

Other than that there are only minor scraps about him. There are a few scripts of a city founded by a cult who worshiped him. Due to changing geography it is hard to tell where it is at this point, but it's easy enough to deduce that it's on one of the islands in the east ocean. There are a few legends of mortals who entertained him gaining great gifts without a usual fiendish trickery attached, and other legends that say he has a fondness for Plantlife, Rubies, Tomes of Knowledge, and Alchohol. Sucking up and Flattery are also effective.