He had been worried about this regeneration since playing Eighth Man Bound as a child, where he saw a great darkness lying after his seventh face. Now he understands why. But the darkness is not merely over him, but over the whole planet.
There is no way to cast off this shadow. Peter Pan may have lost his shadow once, but that was The Boy Who Never Grew Up, not The Doctor Who Never Died. Of all the human stories he'd read, that one stuck with him, because Peter was like him: stuck in between two worlds. Not quite human.