The story is basically Ahriman finding himself. After the Rubric he is a shell of what he once was, destroyed by the knowledge of what he has done to his brothers. He is serving a no-name Chaos Lord Gzrel and the Harrowing warband under a false name. And he is playing second-fiddle to a weak coven of Sorcerers with minor talents at an exaggerated best. That changes when a Thousand Son arrives looking for him, and Ahriman is forced to kill them all. He then takes over the ship Titan Child that was recently hijacked by the warband and allies with a group of 3 Space Marines from a dead chapter that isn't revealed in the book. Their leader, a Librarian named Astreos, is compelled to give an oath of service to Ahriman after he helps them escape.
The story follows Ahriman as he searches for answers as to what is happening, why the members of his Coven are being abducted or killed, and eventually after finding the last member besides himself that has not disappeared, learns that Amon is behind it. Magnus's old equerry was a member of the Coven and according to Ahriman was the hardest to convince. Ahriman narrowly escapes a trap that Amon leaves for him in a space station where Ahriman looks for answers to finding Amon, one of the renegade marines stays behind to cover their escape and another is mentally wounded and emerges as a different man, and then realises that he has no choice but to confront Amon and learn why he is being hunted. But their navigator has been killed and they are forced to raid an Inquisitorial ship and steal theres, and while there Ahriman sees that the ship was a scout ship for data on the Eye of Terror and sees what they saw. The Planet of the Sorcerers was destroyed.
He realises that Amon did it but learns that because of the time distortions in the Warp that it has not occured yet. But it will unless Ahriman stops him. He and his renegades travel to Amon's growing fleet but they are betrayed by Carmenta, the mistress of the Titan Child, when she tires of Ahriman putting her ship at risk. Ahriman is captured by Amon and learns that Amon has given into despair, planning to kill the Thousand Sons to spare them the horrific fate that Ahriman put onto them. Ahriman, after some convincing, shares his knowledge of the Rubric with Amon so that he can undo it. Astreos and the renegades, using a Daemonhost created by the sole survivor of Gzrel's coven, assault Amon's ship, Sycorax, and rescue Ahriman who after journying into his memories and conversing with his brother Ohrmuzd realises that fate is what you make it, decides that he cannot allow Amon to destroy the XVth Legion. He uses his powers to usurp the Rubric Marines aboard the ship and confronts Amon, telling him that he has given into despair and has forgotten hope. The two fight and Ahriman uses the Rubric to turn Amon to dust, and then takes his armour which is Ahriman's signature armor.
He tells his brothers that he is sorry, and that they can now begin again. The epilogue shows that Ahriman has taken control of Amon's warband, has come to terms with what he did and is determined to start over and has forgiven Carmenta, who is now the mistress of the Sycorax, and that Astreos has become Ahriman's apprentice. It also reveals that the Daemonhost that the surviving Sorcerer Maroth bound is more intelligent than it appears, and that it and Maroth are playing a behind the scenes game and wanted all of this to happen. The book ends here.