So I purchased the new Necron book on Saturday because my wife has a Necron army gathering dust on one of the shelves of the closet in our office. No, really I bought it for her. Honestly (not really).
I got to sit down Saturday evening and do a good read through of the fluff because I knew it was going to be a departure from the 3rd edition fluff, and here is my summary of it.
60 Million-ish years ago the Necrontyr were on their home planet that had a very messed up star. This caused the life expectency of your average Necrontyr to be very veriable, but usually very short. So they developted this emo-like fetish with death and the afterlife.
As the Necrontyr empire spread out to occupy most of the galaxy they became very factious, and the ruler of the empire, known as the Silent King, decided that they needed a cause that the entire race could rally behind, so he picked a fight with the Old Ones. Bad idea.