Age of Darkness is what Forge World calls their 40K spin-off set in Horus Heresy. Naturally, everyone just calls it Horus Heresy. 8th edition dropped last year and Age of Darkness found itself without a rulebook. After all, the game was using modified 7e rules. This forced Forge World to release its own standalone rulebook. However, Alan Bligh, the lead writer for the project passed away from cancer a few months before 8e, and it was a real scramble to get it published. Took them half a year, but now we have a book. And here is my copy of Age of Darkness rulebook!
If you want to know more about what I think about the Horus Heresy ruleset, I have written an article about it!