I would presume, from what I've read here, that Dariel is the previous hero's son...
Why was he taken by the demons ? Maybe so he could be used as leverage ? So that one day, the demons would have some way to make peace demands... Or something like that ?
But since Dariel turned out to be so capable albeit his inability to use magic, he became even more valuable to the demon race.
=> Some of them even liked him as a person.