Cavalier of the Abyss

Cavalier of the Abyss is the sequel to Immortal Regis. The main character is a teenager named Xix who is seeking revenge for the loss of his childhood friend, Siana, against Regis Nex. Regis Nex is really Jae Hyuk Chae, who has become a cold-hearted individual. What caused his transformation from a naive teenager into a brutal king? What happened to Xix and Siana?