The Horus Heresy is not advancing to 8th edition, but instead will have it's own dedicated rulebook that is coming soon.
via Warhammer Community
https://www.warhammer-community.com/2017/06/26/a-new-rulebook-for-the-horus-heresy/
With the new edition of Warhammer 40,000 just out, we’ve had loads of questions from you on what’s happening to your Horus Heresy armies. Since Betrayal, many of you have built Legions of your own and played through stories of bloodshed and treachery. At Warhammer Fest this year, we announced that a new rulebook will be released soon to let you continue playing games in the Age of Darkness. The game you love is staying the same – the aim of this new rulebook is to make life easier for players, whether they’ve been playing from the very beginning or are just making their start in the 31st Millennium. We caught up with Neil Wylie – the lead writer on the Horus Heresy rulebook and writer of several previous Horus Heresy books – to give us a first look at the updated edition.
Neil: There won’t be any major changes to the game. Instead, the rulebook is designed to be a better resource for Horus Heresy players dedicated to your needs during a game and filled with information relevant to you. A good example of this is the Special Rules section; rules not present in the Horus Heresy like And They Shall Know No Fear won’t be featured, while common rules like Legiones Astartes and Agile will be, making this section, and the book as a whole, much easier to use as a reference guide for Battles in the Age of Darkness.
The book will also include rules from now out-of-print expansions like Stronghold Assault in the book. You’ll be able to protect your Legions with Aegis Defence Lines and other fortifications from the previous edition of Warhammer 40,000.
If you’ve never tried Battles in the Age of Darkness and have always wanted to, this book is your perfect way to get on board, while seasoned players finally have a rulebook exclusively for the Horus Heresy.
In short, you’ve got nothing to worry about – if you’re already playing Battles in the Age of Darkness, then the new edition won’t significantly change your experience – your books and your armies will still be usable when the new rulebook lands.
With the ever expanding range of Horus Heresy miniatures and a new rulebook on the way, now is a great time to jump into gaming in the Age of Darkness.
