It is the 203rd year of the Imperium of Man's Great Crusade, and humanity's star is ascendant across the galaxy. Captain Garviel Loken of the Luna Wolves sees himself elevated to the Mournival for his actions on Planet 63-19. But what he witnessed on that world, and on his next two engagements, will alter the destiny of the galaxy… and his legion and Primarch.
- Absurdly Sharp Claws: Megarachnid claws are sharp enough to slice through Space Marine Powered Armor like tissue paper. Lucius gets his hands on a severed claw and starts using it as an improvised sword to deadly effect.
- Arrogant Kung-Fu Guy: Lucius is one of the finest swordsmen in the Imperium, and he isn't shy about letting others know.
- Bitch in Sheep's Clothing: Erebus is initially shown as someone who gains the immediate respect of the Mournival and becomes a close advisor to the Warmaster. But towards the end it is shown that he was the one who stole the anathame from the interex and kicked off the war between them and the Imperium.
- Bug War: On Murder, the Luna Wolves and Blood Angels get into a war with a race of vicious arthropods called the megarachnid.
- The Cameo: Dorn was present for and observed the conquest of Sixty-Three-Nineteen as a guest of the Luna Wolves, though he and his contingent of Imperial Fists didn't participate. He gives Loken a few words of encouragement before he heads off to Terra for other duties. Sanguinius later showed up to co-command the conquest and cleansing of Murder. These were particularly poignant because they were Horus's closest brothers, and, beside the Emperor, would go on to become his chief antagonists during the Heresy. Though in Dorn's case, their relationship would be more of an Informed Attribute.
- Curb Stomp Cushion: While the Space Marines' victory over the "Emperor" of 63-19 is swift and brutal, they do take losses when storming his palace. This shows straight off the bat that as powerful as Astartes are, they are not invincible.
- Electromagnetic Ghosts: Or daemons, in this case. Samus first makes his presence known by whispering his Madness Mantra over the Imperials' vox network, unnerving all who hear his words.
- Elite Mooks: The "Emperor's" Invisibles are elite guards armed with energy-manipulating technology that can both make them literally invisible while they're standing still, and punch through Space Marine armour when used offensively.
- Establishing Series Moment:
- The Luna Wolves charging into battle backed up by the Legio Mortis immediately tells readers just how much action each book is going to have.
- The scenes with Samus remind us that, however awesome the series is, it is still a Cosmic Horror Story.
- Foil: The interex are one for the Imperium. They are an advanced human state that has learned to peacefully coexist with xenos and are perfectly aware of and on guard against the dangers of "Kaos".
- Foreshadowing: The book is jam-packed with it.
- Abaddon proclaims the ruler of 63-19 to be a "false Emperor".
- The Justaerin Terminators are said to look like "they were from some other, black Legion."
- Sigismund of the Imperial Fists dourly predicts that there will only be war in humanity's far future and is met with amused disbelief.
- Luc Sedirae says early on that "The only thing that can beat an Astartes is another Astartes". The narration goes on to say 'and they had all laughed at that. The impossible was nothing to be scared of.'
- Loken regretfully muses that there will be a time when Abaddon no longer wages war across the galaxy.
- Four-Temperament Ensemble: This is the point of the Mournival. They are meant to balance each other's temperaments out so that Horus gets the best advice possible.
- Glad I Thought of It: Eidolon takes credit for Tarvitz's blowing up the trees on Murder as soon as it becomes apparent that it wasn't just a waste of precious explosives.
- Glory Hound: Lord Commander Eidolon of the Emperor's Children is an unrepentant glory-seeker who leads his men into a death trap on Murder and refuses to take blame or share credit. It quickly gets him on the Luna Wolves' bad side.
- Hearing Voices: The most unnerving thing about Samus' whispers is that even people who don't have voxes can hear him.
- Hope Spot: While conversing with Loken, Mithras Tull still realizes the Luna Wolves don’t actually know about Chaos, and the two finally seem to find common ground to stand on. Unfortunately for them, Erebus, who IS in service of the Dark Gods, chooses that moment to ruin everything.
- Impaled with Extreme Prejudice: On Murder, Saul Tarvitz discovers that the megarachnid have been impaling the Blood Angels on the branches of a giant stone tree, like shrikes impaling mice.
- Improvised Weapon User: Lucius uses a severed megarachnid limb as a sword until Eidolon tells him to throw it away.
- Intrepid Reporter: Euphrati Keeler wades into the middle of the Crusade on 63-19 and almost singlehandedly redeems the reputation of the remembrancer order with her photos.
- Madness Mantra: "Samus. That's the only name you'll hear. Samus. It means the end and the death. Samus. I am Samus. Samus is all around you. Samus is the man beside you. Samus will gnaw upon your bones. Look out! Samus is here."
- The Muse: Loken becomes Mersadie Oliton's muse, as she begins developing a documentary of the Crusade through the lens of his experiences.
- Not-So-Small Role: Erebus being present with the Luna Wolves. His bit part in this book is stealing the athame blade during negotiations with the interex, dashing the high hopes of the peace talks by causing them to break down. A small war broke out which lasted throughout the night before the Luna Wolves could escape, and expressed what little good will they could salvage in the situation. Erebus will go on to have a pivotal role in several events in the Horus Heresy, most importantly in flipping Horus to Chaos.
- Outgrown Such Silly Superstitions: The Imperial Truth is still going strong here, with many believing that gods and daemons are myths. Samus begs to differ.
- Outside-Context Problem: Samus ends up being this for Loken and the Luna Wolves in the Whisperhead Mountains. Unlike every other foe the Astartes have faced up to this point, Samus is a genuine daemon, with outright supernatural abilities, who manages to kill several Space Marines with relative ease and shrug off attempts to kill him. His presence ends up setting the stage for much worse things to come...
- Planet Terra: It starts with the fleet finding a planet called Terra whose inhabitants claim is the original Earth.
- Poor Communication Kills: The interex kept the Luna Wolves at a distance because they were worried that the Imperial forces had been tainted by "Kaos." It's only moments prior to Erebus raiding the Hall of Devices that a member of the interex realizes that the Imperium doesn't even know what Kaos is. If the interex had been a bit more open with Horus at the start, it might have been possible to prevent the Luna Wolves from falling to Chaos...
- Reasonable Authority Figure: Horus goes to great lengths to establish himself as one. He personally comes down to reassure Loken after the debacle in the Whisperheads and frequently uses the Mournival as his attack dogs so that he can rein them in and look like a conciliatory figure. He also decides to talk to the interex instead of just exterminating them wholesale, hoping that the Imperium can learn from them.
- Small Role, Big Impact: Erebus only shows up for the last third of the book, but his actions lead to the war between the Imperium and the interex, and set the stage for Horus to fall.
- This Cannot Be!: Loken is completely stunned when Xavyer Jubal turns on him and his other Astartes and starts killing them. It takes him several moments even to grasp the idea that a Space Marine could do such a thing.
- Too Dumb to Live: Remembrancer Borodin Flora decides the best course of action when Jubal mutates into a massive, deadly beast is to walk up to him and tell him to begone while making the sign of the Aquila. His reward is to summarily be bitten in half.
- Violence Is Disturbing: The remembrancers are equal parts horrified and nauseated when they get to see the gory aftermath of the Luna Wolves' clearing out of a rebel stronghold in the Whisperhead Mountains.
- Wham Shot: The penultimate portion of the story shows Erebus examining the anathame, revealing that he was the one who stole the blade and started the war between the Imperium and the interex.
