User blog comment:0770/Frau's darkness/@comment-5714639-20121202065444

Rather than it being his darkness that's deeper than that of his fellow Ghosts, I think that it's Frau's pain that might be deeper.

When Labrador's uncle prophesized that Frau would disappear from the world, I'm guessing he means that Frau will be killed. If Frau is killed, then most likely his soul will return to heaven, and therefore will no longer be in the physical world. Frau is unimpressed, and is actually very accepting of the fact, which means he's already come to terms with the fact that he will die before Lab's uncle told him.

Frau devouring Marie's lover was actually part of his job as one of the 07-Ghosts, which used to be Verloren's job. When Frau saw that Marie's lover couldn't be saved due to him being hated by heaven, Frau devoured his soul as punishment for the man's sins.

When Frau says, "You and I will fall together to hell," it's making a clear point: Frau believes he's also hated by heaven due to him wielding Verloren's scythe and believes that by devouring Maries sinful lover, he is just as bad a sinner as Marie's lover.

I've always suspected that Frau wanted Mikhail to kill him because Frau can then pretend that Teito is the one killing him. After all, Mikhail possesses Teito and the only thing that changes is Teito's eye color, so Frau could easily imagine that Teito is killing him. However, since Frau didn't really know how to ask Teito to kill him, he settled for requesting to die at Mikhail's hands, which would be as close to Teito killing him as he could get. Later, Frau manages to admit to Teito that he would like to die by Teito's hands, if the situation calls for it.

As for the reason for Frau's pain, that's a bit more complicated. Frau, whose companions died during the Sky Pirate subjugation, obviously feels guilty that he is still "alive" while his companions all died. He mentions twice throughout the series that his god is dead, and the only light in his life comes from Teito.