Horus has just been named Warmaster as the Emperor has decided to return to Terra (Earth) to let Horus prosecute the Great Crusade as he sees fit. But this new found power and prestige has started to change Horus as his idealism begins to be slowly warped.
As the short preface states:
"The vast armies of the Emperor of Earth have conquered the galaxy in a Great Crusade - the myriad alien races have been smashed by the Emperor's elite warriors ... First and foremost amongst these are the primarchs, superheroic beings who have led the Emperor's armies of Space Marines in victory after victory. They are unstoppable and magnificent, the pinnacle of the Emperor's genetic experimentation. The space Marines are the mightiest human warriors the galaxy has ever known, each capable of besting a hundred normal men or more in combat ... Chief amongst the primarchs is Horus, called the Glorious, the Brightest Star, favourite of the Emperor, and like a son unto him. He is the Warmaster."
The parallels with Milton's Paradise Lost are very evident, but Abnett does a good job of not making it seem like a simple retelling.
Overall Horus Rising is a pacy story that left me wanting to read the rest of the books in the series. Fans of Warhammer 40,000 will enjoy the background material these books contain, and fans of military science fiction will also get a kick from this book.