
What the hell does this story have to do with immortal regis? I JUST WANT TO KNOW IF THE BOY SAVES HIS BROTHER DAMMIT!

To be make you understand, Cavalier of the Abyss is a sequel to Immortal Regis. The main thing in this story is mostly about... (this might be some spoilers)
The main character of Immortal Regis have children and haven't save his younger brother yet even after many years have passed. Cavalier of the Abyss is mostly focused on Xix, who is his son that he didn't know since Serin hid her children to prevent dangers toward Xix and Siana.

That's what a prequel is, it sets up the characters and place for the sequel. Plus a lot of the characters from Immortal Regis reappear in Cavalier of the Abyss, but only older since time has passed.
Unhappy beginning, sad middle, happy ending. :)