I think everyone that is a power armor and fluff nut like me (I am hoping that there are others out there like me) knows about the Legion of the Damned. The fluff says that one of the 21st cursed-founding chapters, The Fire Hawk was sent to fight Eldar pirate. The entire chapter made a warp jump to Crowe's World, but none of them arrive (I personally blame those dirty Eldar!).
After that, from time to time, when Imperial forces were facing insurmountable odds, the silent Black and Flame armored Astartes would joint the fight and leave without a word. These were the Legion of the Damned.
When I saw that this book was coming out I got very excited. I like any of the "mysteries" in 40k, and this is one. "Too bad the book was coming out in April", I thought to myself.