I have a feeling that Lispen was like professor Heil, a half-human, half-demon. that's why his blood was black. And that Lidusis' brother was in cohort with the black mana traitor. And the three of them were trying to make Lidusis wake his magician power or something.
I remember this part,reread it just recently, but i think that giant meant that rood is demon. oh, maybe all powerful demons are born from one thing and they are all like brothers or something. But my dictionary said that "brethren" is used for someone part of the same religious brotherhood cult or sect. so I hardly believe that rood has any blood relatives.
OK, so here is what I came up with after reareading the story again. May contain SPOILERS, BEWARE!!!
Blow/Rood is a demon king whose power was supressed by the tower and was turned into a human 8 years ago with that ritual they are talking about. Also the greatest evil is lys`s brother. he must be that shadow. I mean lispen is a demon( look at his eyes) but lys`s brother said that he will punish him. so he must be stronger that lispen. also, didn`t lys have some dark power or something? Oh, and ruby and dio mentioned someone in the wast chapter. I believe that he is lys`s brother. And we musn`t forget the tower who want to catch rood. I mean, the leader said "Your freedeom for the past 8 years has been enough." Maybe, not just maybe Rood`s master has been keeping him a secteret and protecting him from the tower and the others since he had found him. Oh one was thing: when I reread the 0 chapter again there was something... maybe the king had asked for help the king of demons-blow and then after some years the magicians from tower had found a way to cathch that demon king and maybe tried to use him but failed.
all in all, the pieces of the puzze are starting to fall in, but there also so many new questions.
What do you think? Have some theories?