The Heresy officially begins as a fully corrupted Horus unleashes Hell upon the planet of Isstvan III. On the ground, a trapped and desperate loyalist force tries to survive, while those above try and get word to Terra.
- An Arm and a Leg: Nathaniel Garro loses a leg during the fighting on Isstvan Extremis.
- Body Horror: The reader is given vivid descriptions of the Life-Eater virus doing its thing to the populace of Isstvan III and the loyalist Astartes who didn't get to cover in time.
- Character Death:
- Saul Tarvitz and Nero Vipus are killed during the second firebombing.
- Tarik Torgaddon dies in battle with Horus Aximand.
- Defector from Decadence: Iacton Qruze formally renounces the Sons of Horus after witnessing the virus bombing.Qruze: I am no longer part of the Legion. I can no longer be party to what the Warmaster is doing.
- Dude, Where's My Respect?: Lucius gets increasingly upset by Tarvitz's assumption of command; this is part of what pushes him into turning on the loyalists.
- Face Death with Dignity: Knowing there is no hiding from the second firebombing, Tarvitz and the remaining loyalist Astartes face it down with dignity, content in the knowledge that even though they are dying, they still won by bleeding the traitors and delaying their plans.
- Face–Heel Turn: This is where the Sons of Horus, Emperor's Children, World Eaters, and Death Guard officially turn on the Imperium.
- General Failure: Eidolon makes a complete hash of the ground assault on the Precentor's Palace, despite having superior numbers and support from Legio Mortis.
- It's Personal: Loken and Torgaddon turn down all offers of help when they go off to face Abaddon and Aximand, telling the other loyalists that this fight is Mournival business.
- Jumping Off the Slippery Slope: Horus was already teetering on the edge thanks to his actions in False Gods, but he officially hops off it here, as summed up in one quote as he virus bombs Isstvan III.Horus: Order the guns to fire! Let the galaxy burn!
- My God, What Have I Done?: Little Horus is completely shattered after slaying Torgaddon.
- No Kill like Overkill: The loyalist forces prove so stubborn and slow down his plans so much that Horus eventually just firebombs the planet again to finish them off.
- Oh, Crap!: Tarvitz is horrified when he realizes that the fleet is about to destroy Isstvan III with virus bombs. He then runs for his life when he sees Angron make planetfall.
- O.O.C. Is Serious Business: Tarvitz first realizes something is wrong when he finds out that Eidolon isn't leading the Emperor's Children deployment to Isstvan III, a highly uncharacteristic move for him.
- Spanner in the Works: Three big wrenches get chucked into Horus' plans in rapid succession:
- Saul Tarvitz manages to get word of the incoming virus bombs to the loyalists groundside, allowing many of them to take shelter. He also manages to convince Nathaniel Garro to side with him, which as the next book shows is what reveals the Heresy to the rest of the Imperium.
- Angron then blows Horus' plan to wipe out the survivors via a second orbital bombardment by taking his Legion down to Isstvan to kill the remaining loyalist World Eaters personally.
- Iacton Qruze, the ignored "Half-Heard", sneaks Keeler and company off the Vengeful Spirit to a loyalist Death Guard ship, which then successfully flees the system and gets out word of the betrayal.
- Spotting the Thread: Invoked and subverted. Most of the loyalist characters notice that something is off when they find out that the Isstvan speartip is being assembled apparently at random, with squads and companies plucked out of their usual spots and deployed willy-nilly, but they just shrug it off as being the Warmaster's prerogative. Saul Tarvitz is the only one who realizes that something is very wrong because he also notices that Glory Hound Eidolon isn't leading the Emperor's Children contingent.
- Super-Scream: Eidolon uses his newly enhanced vocal cords to stun a Warsinger on Isstvan Extremis long enough to kill her.
