What is wisdom without debate? Why condemn everyone around you in order to punish yourself? This book, which should've been named A Thousand Bad Decisions, all made with the absolute best of intentions, is Ahzek Ahriman's attempt to set the record straight about what his father did and why. In the 41st millennium, Ahriman is one of the most murderous sorcerors to ever plague the Imperium, but 10,000 years before that he was a loyal son and devoted scholar.
Long before the dropsite massacre, Amlodhi Skarssen Skarssensson, high jackass of the Wolf Wolves, journeys to the planet Aghoru to demand Magnus, primarch of the XV Legion (Thousand Sons), join Magnus' brothers Leman Russ and Lorgar, primarchs of the VI and XVII Legion respectively. Magnus refuses; there is archaeology to be done. Once the Thousand Sons and the Wolf Wolves finish shooting everything, it's time to head to the Ark Reach Cluster.
The citizens of Ark Reach made the mistake of not bending to the Emperor's will immediately and it is time for them to die. Leman Russ murders everyone, including those trying to surrender. Magnus is impressed by their technology and tries to stop the Wolf Wolves from destroying a library. Leman Russ and his Wolf Wolves howl like wolves to express their rage, but Magnus will not be denied access to books. Only the timely intervention of Lorgar, the Urizen, prevents space marine from fighting space marine.
With Skarssensson is Ohthere Wydrmake, a Wolf Wolves Rune Priest. Wyrdmake befriends Ahriman and Ahriman teaches the ignorant barbarian shaman the wonders of The Great Ocean. But Wyrdmake's friendship is a ruse! Leman Russ never forgot that one time Magnus stopped him from murdering everything, and the Rune Priest's real mission was to gather evidence against the Thousand Sons!
That evidence is presented at the Council of Nikea, a literal witch hunt. "No more sorcery!", the Emperor commands! The Thousand Sons, an entire legion of psykers (sorcerers), are forced to return to their homeworld of Prospero, to think about what they've done.
But but but! "We have done no wrong!" Magnus reasons. And The Emperor is in great danger from unexpected treachery! Surely The Emperor will forgive Magnus if he uses sorcery to warn him of that treachery??
And everything falls apart.
The Thousand Sons were loyal to The Emperor and the Imperium of Man. That The Emperor could err so disastrously as to unleash the Wolf Wolves, his executioners, upon the Thousand Sons has never been convincingly explained. Even now, this book is only one half of the story, and it creates as many questions as it answers. Tizca, the City of Light, was a glimpse of what the Imperium of Man should have been. Its citizens were a vision of what mankind could have become. If only The Emperor had completed the Golden Throne! If only Magnus had listened to his father and assumed his place on the Golden Throne to guide humanity to its shining future! If only! If if if...
If there exists a better written tragedy, then I haven't read it. The Thousand Sons should've stood shoulder to shoulder with their brothers, the Imperial Fists, on the walls of the Imperial palace. Hell, with the Thousand Sons firmly on the loyalists' side, Horus and his goons probably wouldn't have made it that far. I've said it over and over with every book in the Horus Heresy, and I'll say it again: it didn't have to be this way.