Its pretty old and it has two parts
In the prequel two humans are taken to the demon world from a witch the younger brother was sick then got possessed.
In the later one the older brother was demon king and his son was trying to kill him for sealing his mother the witch who originally took him.

Not completely sure, but your description sounds like Immortal Regis, the prequel, and Cavalier of the Abyss, the sequel. Hopefully it's what you're looking.