not a chapter as such, I believe they're the guardians of the emperors palace... or maybe just Terra. The elite of the elite. I think the best marines from each chapter are chosen to be part of it, but don't quote me on that.
I read it because I play a war game but nothing like you guys are talking about. It's a computer game... it's old, from way before World of Warcraft, it was the first multiplayer game with one million players.
Video!
You build camps and command the robots (called rovers) in attacks of defenses of camps... you have a group called an MDN who protect you and you wage wars with them. Pretty fun stuff. It's a first person shooter/strategy war game.
Couple screen shots:
The person who owns the yellow rovers just got owned in that last pic.
NMM is always a winner, I should practice that someday. Using metallic colors is pretty easy though and I usually paint fast to get my army playable instead of super good looking.