The legends go that the first incubus was born of a demon and angel. Abandoned by both parents on earth, he became jealous and searched for a way to increase his power so as to destroy them. He found that through sex he was able to absorb the life force of a human.
“Angels are bright still, though the brightest fell.Though all things foul would wear the brows of grace,Yet Grace must still look so.” It means that even though all angels are good, the best one still fell ( lucifer), but that doesn't take away from the rest being good. A way of saying do not generalize things because the worst can happen to the best of us.