Kaeros was an Elder Breed dragon of the Order of the Flame, Bonded with Navaros. When Navaros turned against the Order, Kaeros apparently followed willingly. When he founded the Dark Union and turned other dragon's against the Order, Navaros grew resentful of sharing power with the dragon and used dark magic to absorb his very soul, making Kaeros' body effectively his own. When Navaros' human body was destroyed and his soul flung into the Rift Gate, the soulless body of Kaeros went with him.
Kaeros returned 600 years later along with the newly resurrected Navaros. His body was significantly larger and he now had four heads. This is possibly due to the corruption wrought upon his soul by Navaros. After Navaros' soul was banished from the body of Rynn's brother, Delon, he resumed possession of Kaeros' body in an ill-fated, last-ditch attempt to kill the warrior woman and her Bonded dragon, Arokh.
Drakan: Order of the Flame - Drakan: The Ancients' Gates