The Demons blessing gives some sort of great power to its recipient. This corrupts their nature into something akain to a demon, and grants them demon-like power.
Raven was finally able to rid himself of its influence after Goldas Arenian defeated him at the end of Imaginaughts Nex. Raven was already a demon by then, so he did not experience any loss of power, only a change in personality.
It appears as an ordinary burn or cut, but refuses to heal. Raven's is a claw mark across his back.
More will be added.
Raven was the first to receive the "blessing" at his age (roughly 7 in Aefian years).