The Horus Heresy is to deal with the legions at the time of the fratricidal war nevertheless, this is not the case here. The first 300 pages tell of the riders on the home planet Caliban, only in the last 100 pages diving Space Marines on. And then everything seems hasty, as if they wanted to quickly squeeze in yet something with demons creatures in the book.
The name Horus falls in the book not even that heresy is not subject. The story would have worked very well as a single point outside the Horus Heresy series, then you might have had false expectations.
But all in all a boring book that is the rest of the series does not do justice. Can I leave out confidently, you will not miss anything.
And that's why I had considered giving him even 2 stars, but as my German teacher once said has "this is clearly missed an issue since you can not be anything other than a 6".