If you don't want to listen to my little yapping at the end, i'm making a guess that on chapter 45 in Dark Fall explains why Nergal and Cliff don't get along. It was while speaking with Mephisto about the "Child King Rubierre" how i think the "cursed dragon" is Cliff (because on chapter 68 at the end, Cliff shows Leon his true form is a dragon) and the child is Nergal.
(also sorry for the background music, don't want any copyright problems...
-_-)