It seems the leaks are flowing from White Dwarf's like crazy atm. A codex Legion of the Damned is also leaking, with pics to show that its real. This is going to be a little crazy in March with two mini-dex's hitting.
Here are some links to the leaked images. The guys at 40k radio apparently have a first class link to early White Dwarf images.
https://www.facebook.com/pages/40K-Radio/147396461962884
Of course we dont know much yet, except that there are rules to use them as allies for any imperial army, and specific scenarios as well.