I would love if anyone could name me this manga which I can't find! It starts with a little boy who sees his grandfather's soul being taken by a demon, but because of the grandfather's last words, isn't scared of him. The demon is amused by the child and takes him back to his castle (he's a demon lord, of course) and is asked by the child if he can stay there with him forever. The demon's kind of evil and is about to take advantage when he is persuaded to let the child make the choice when he grows upu a bit more/in the future.

Fast forward about 8 years, and the kid's got a big brother figure and a best friend who are both concerned that something bad is going to happen to the kid. Meanwhile, the kid himself is psyched that the demon has contacted him about returning, even though there are super sinister implications that he's going to have to do something terrible/affect his family unit with a lot of pain to "prove" he wants to be with the demon.

Anyone know what I'm talking about? Not being able to find this is driving me crazy!

That would be Vartrag Tale.

That's perfect! Thanks so much!