"He sucks the very brightness out of the place," Michael says when Castiel finally summons the courage to ask him what Lucifer is like. He's relieved that Michael isn't mad. "It used to be a good thing; it used to make him shine. There was no brightness without him, that wasn't a part of him. When he Fell, he didn't create a cluster, like the others; he left the first black hole. Every black thing in him steals all the light." Michael's armor clinks together, and it occurs to Castiel that he's never seen Michael without it. "He is the most beautiful thing Father has ever Created, and ever will Create, but he is also the most foul." He looks right at Castiel, and Castiel can understand why Michael is the oldest, why he's in charge. "He captures you, and you can't escape; no angel, no shred of light, no thought or idea. He is a vacuum, Castiel. He takes everything, and gives nothing in return."
Castiel listens quietly, and when Michael is done, he remembers what Uriel told him, that Michael hasn't smiled since Lucifer Fell, not really, and he figures Lucifer stole Michael's brightness, too.