Copied from Reddit to remind me what happened:
[Spoilers] Siege of Terra: Lost and the Damned Full Summary
In my tradition of summarizing the events of HH books, here's the Lost and the Damned full summary:
*Book starts off with the traitor fleet bombarding Terra from Orbit. The first shot is fired directly at the Sanctum Imperialis, but does no damage and the shell is intercepted by dozens of point defense lasers before it even reaches the void shields. The loyalist guns on the ground respond, and a massive exchange of fire takes place. However Horus' bombardment is largely inefficient on the Palace itself due to "The Aegis", an Dark Age-era void shield array far above anything else the Imperium has. The Aegis is multi-layered and self-repairing, and in concept is invincible. Horus could simply exterminatus the planet, but doesn't due to the importance of Terra to ruling the Imperium. Dorn has also placed Terra's last orbital place in the atmosphere over the Palace, providing a sort of sacrificial shield as well as gun platform
*Dorn is basically forcibly conscripting anyone who can hold a lasgun, sending them in cramped trains around the Palace. There's an "acting officer" (implied to be a regular Imperial Army professional) for every 300 soldiers and anyone who stumbles off or tries to desert is executed. Our main character for these characters is Katsuhiro, a Japanese ("Dragon Nations") enumerator who's in way over his head. Their training consists of the forced multi-kilometer march from the train dropoff point to their post, which is considerably long. Along the way they're taught to roughly march in formation and told to aim and pull the trigger on their lasgun. Katsuhiro is posted on the 1st of 3 trench lines around the outermost palace wall, set up to avoid Chaos forces from being able to reach and dig under the walls themselves. We meet several of Katsuhiro's squadmates, 1 of which is an Alpha Legion operative from earlier book Myzmadra and the other a grizzled veteran called Doromek who tried to intentionally avoid the earlier drafts. The desperation and grimdark style of these chapters reminds me of 15 hours.
*For symbolic/morale purposes, Dorn holds a regular meeting with the High Lords of Terra in the Great Chamber of the Senatorum Imperialis as the bombardment rains down against the Aegis. They all discuss the situation with Dorn, the Khan, Sanguinius, Valdor, and Malcador. Dorn emphasizes the main strategy is to buy time for Guilliman to arrive and all efforts are designed to bog down Horus for as long as possible, which is why he's doing things like forcibly conscripting hundreds of millions of souls. The forced conscription and their deployment outside the palace in what is essentially a suicidal mission rubs Khan the wrong way. Dorn also states that if he wanted to run a rebellion he would have skipped Terra and tried to have taken as much territory elsewhere as possible, waging a war in Segmentum Pacificus to bleed out the Imperium's resources and eventually encircle Segmentum Solar. The Terra rush is because Horus is being driven by the Chaos Gods to slay the Emperor personally, the Gods care about nothing else but his death. Unlike earlier books, the loyalists seem to have a full understanding of the Warp here and never underestimate or question its malice or capabilities.
*A weary Malcador for once in his life finally opens up about nearly everything, and honestly. He tells the Primarch's about the Webway Project, the need to hide Chaos, that it was a mistake to not tell the Primarch's the truth, the Webway being of Xenos origin, etc.. He updates on the Primarch's situation, telling Khan/Sanguinius/Dorn that Vulkan lives and is inside the Imperial Palace looking after a failsafe by the Webway Portal. There has been much discussion on this scene, because Dorn apparently is shocked despite having earlier met Vulkan in the novel Old Earth. I'm not sure what Haley was doing here, but it should be noted that Dorn simply reacts with a shocked "what?!" when Malcador says Vulkan Lives, Sanguinius/Khan ask all the other questions.
*Aboard the Vengeful Spirit, Abaddon has become disgusted with Horus. He is literally swelling with power, his face and form bubbling as the Chaos Gods channel massive energies into his body continually. He spends most of his time sitting in a comatose state on his throne, clutching the skull of Ferrus Manus. Lupercal's Court isn't just physically corrupted, it now exists in a warp pocket dimension inside the ship itself. When Abaddon steps into the chamber it's like he's stepping foot on a Daemon World. Horus is guarded by only 2 Justaerin and Falkus Kibre, but hundreds of Word Bearers under Zardu Layak are in the room. This is something that disgusts Abaddon greatly and he continually resists the urge to kill Layak. Abaddon says Horus' old charisma is appearing less and less, and he is more of a sort of terrifying abomination than anything else now that commands through fear instead of personality. Abaddon wants Horus to wake up and command the siege, but Layak says he's doing important work as his spirit walks the Warp.
*We see what Horus is up to inside the Warp in a rather surreal sequence. It's a flashback to his first time seeing the Emperor, when he was young and brought from Cthonia to Terra to much celebration. As his dropship descends, he's accompanied by 4 old comrades that over the course of the conversation are revealed to be the Chaos Gods (this is never stated directly, but simply becomes obvious by their words and descriptions of their appearances). Horus is not entirely lucid and can't really remember who these companions were, nor can he remember their names. Throughout it all all 4 companions urge Horus on, celebrating his achievement and status. Horus lands and marches across a large avenue before a throne that the Emperor sits upon, millions crowded on both sides cheering him on. The 4 companions follow him but suddenly shrink back. The Emperor speaks, stating that it's not enough Horus torment him in the present but must also ruin their old fond memories of the past now as well. The conversation between the "present" Emperor and Horus plays alongside what actually transpired, and as you see Horus pledging an oath to the Emperor the current Horus is fanatically yelling that he will destroy him. Horus wakes up and nearly collapses, and Abaddon is disgusted by the weakness. He states that he has what he needs but is angry that Mortarion is late.
*We get a scene where Sanguinius flies over the Palace on his own against the wishes of Dorn. During his time over the Palace he suddenly gets a vision seeing how the palace looks in M41 before it snaps back to the present time. He meets with Raldoran and Maximus Thane of the Imperial Fists (from the Beast Arises) at Bastion 16, which the loyalists believe will be the first site the traitors touch down at. It also is where poor Katsuhiro is stationed outside of.
*There's a meeting of the traitor primarch's in space that shows what a mess they've all become. Kelbor-Hal is also there, but is largely shown to be prideful and useless. Perturabo is bitter that Horus is having him simply fortify the sol system in preparation for Guilliman's arrival instead of commanding the siege. Angron is a monster that simply wants to immediately attack. Fulgrim gets his kicks deliberately trolling angron and watching the reaction. Magnus keeps to himself and refuses to really cooperate with anything. All the traitors are projected via hologram inside the Vengeful Spirit. Abaddon, Tormageddon and Aximand are there too, but Horus is late and Abaddon expresses his frustration that he must be with Layak again. Abaddon tries to get the primarch's under control but they laugh him off and Perturabo states that Horus values his brothers more than his servants. Abaddon eventually cuts off Angron's audio feed, so all you see is his hologram screaming which Fulgrim finds hilarious.
*Horus eventually arrives and is able to control the situation, stating that Abaddon has all his authority when he's not around. Layak explains that the Daemon Primarch's are literally being forced into obeying Horus now by their Patron Gods. Perturabo gives a briefing of the situation, stating that despite his duty of fortifying the sol system he also has been analyzing the Aegis and found a weak point with his bombardment patterns. It's centered around Bastion 16, which had a reactor accident recently. The aegis' self-repair ability is weaker there and will take some time to repair. If the traitors concentrate orbital fire on that spot, they can weaken the barrier to the point where slow-moving aircraft can safely enter it and make attack runs at the void shield generators, guns, etc.. Angron doesn't like this, and simply wants to attack the palace immediately, but Magnus explains that the Emperor's psychic might is not only preventing any daemonic manifestation on Terra it also would disintegrate the daemon primarch's if they tried to set foot on its soil. The Emperor is getting weaker, and Magnus believes that by decimating enough population centers elsewhere on the planet they can use the bloodshed to power a ritual to disable the Aegis and the Emperor's psychic shield.
*Is a scene where Sanguinius meets with the loyalist Fabricator-General Kane deep underground, where they show the Legio Solaria Princeps Eshin from Titandeath. She reveals that she saw Horus collapse on Beta-Garmon. The information really leads nowhere for this book however so it's sort of a useless scene.
*Moves to the first traitor landings on Terra, which are conducted by the Lost and the Damned. Specifically Traitor Army who have been sitting in their ships for months doing nothing and a herd of Beastmen which are driven into a frenzy of a recorded speech by Layak where he points out the injustices the Imperium has heaped upon his kind. Despite being for void use only the traitors forcibly land these massive troop transports around the palace, specifically Bastion 16. Hordes of Beastmen, Mutants, Traitor Army, & Cultists attack the panicked conscripts but they repel the attack. This pattern repeats itself many times over the coming weeks. During one such battle one of Katsuhiro's comrades who accused Doromek of being suspicious is found shot through the back by a lasburn.
*As the traitors make their landings, it changes perspective to a pilot of a Imperial Army fighter wing at the Palace. The traitors make constant bombing attempts on void shield generators, anti-gun batteries, etc and it's a bit of a Battle of Britain scenario. There are no enemy astartes aircraft and the Imperials dominate the fighting, though one by one anti-ship batteries and void arrays are being disabled by weight of numbers.
*There's a bit where Katsuhiro and a few others in his squad venture inside a tunnel that leads back to the Palace Gates, originally built for Officers to flee once their line has been overrun. Myzmarada, posing as an army soldier, is planning to detonate explosives where it leads under Bastion 16. There's a flashback by Myzmarada that shows how she was recruited. A Marine claiming to be Alpharius says that she's the type of person they want, loyal to the ideals of the Imperium but hates how it's being run. He is surprisingly forthcoming about everything, stating that in a few years there will be a rebellion where Horus will go rogue and the Alpha Leigon will join him as Horus' victory will ultimately allow humanity to survive.
*Perturabo meets with Horus personally on the Vengeful Spirit away from the other Primarch's for the next stage of the plan to breach the Aegis. Pert sees Horus as a slave to Chaos, and is quietly gathering all warp lore so he can master the warp unlike Horus has. Horus very much appeals to Perturabo's emotions here, stating that he appreciates all he has done and he's the only other primarch he trusts anymore. He considers the other Primarch's children, but not Perturabo. Pert is somewhat flattered but highly paranoid/distrustful of Horus still nonetheless. The Daemonically Mortarion later arrives and his very presence is killing the chapter serfs and sending Sons of Horus into seizures by the smell alone. Only Horus, Tormageddon, and Abaddon are unaffected as Aximand, Falkus, etc are convulsing (Pert isn't there anymore). Abaddon himself doesn't get why he's unaffected and Layak hints its because he already has the Gods favor, which Abbadon dismisses angrily. Horus tells Mortarion that he'll uphold his vow to have the Death Guard be the first on Terra.
Angron is furious when he hears the news of the Death Guard landing on Terra first while he still does nothing. He goes inside The Conqueror and starts to rampage across its lower decks, massacring anything he comes across. Lotara Sarin appears, haggard and dirty but still rational. She has basically been trying to keep Angron out of the ship for quite some time due to the damage he causes, hence why he has been seen perched on its exterior so much in recent books. She tells Kharn (who himself is barely resisting the urge to massacre Lotara and everything else) that this time Angron's warpath is leading to the reactor core, and if he kills all the Tech-Adepts there they risk a meltdown that would destroy the ship. She reveals she has been talking to acting Night Lords leader Gendor Skraivok, who controls the Nightfall, about putting Angron inside the ships labyrinth until it's safe to unleash him on Terra. Kharn follows Angron down into the bowels of the Conqueror and fights with Angron, who is in such a frenzy that he now wants to take his skull for khorne as it'd make a fine prize. Angron reveals he knows Kharn's ultimate destiny and wants to supplant him as the Chosen of Khorne. Kharn gets his ass kicked until he places a teleport hormer on Angron which sends him into the Nightfall's ever-shifting labyrinth where he will rampage for the next several weeks.
The next phase of the siege begins several weeks after the initial bombardment. Kelbor-Hal's Disciples (Nul Through Oct, led by Sota-Nul) each beach one of their Ark Mechanicus ships around the Palace. If one draws a straight line connecting the Ark Mechanicus to the Sanctum Imperialis, it forms the Star of Chaos. The ships unleash endless waves of Lost and the Damned as their void shields provide cover to begin erecting siege engines. At the same time, the Death Guard begin their drop pod assault on the Walls and the first Chaos Astartes set foot on Terra. Katsuhiro has to deal with it. All seems lost for the group around Bastion 16 until Jaghatai Khan arrives leading a massive White Scars jetbike charge, he refuses to leave the conscripts to die despite Dorn's orders. The charge reaches the Dark Mechanicus landing zones as the Khan slays over 40 Death Guard Marines himself before being stabbed by a warp-tainted poison blade that saps his strength. Sanguinius also violates Dorn's orders and sweeps into the warzone alongside Raldoron and the Blood Angels. Sanguinius rescues Jaghatai, who he finds beneath a mound of dead Death Guard. They lead a semi-organized retreat back to the final trench around Bastion 16 which is basically directly in front of the Helios Gate of the Imperial Palace. As soon as Jaghatai enters the Palace perimeter the Emperor's psychic powers kills the warp taint in his body. During all this the Night Lords launch a Raptor attack on the Palace Walls under Lucoryphus from the ADB NL novels. Despite consider themselves above Chaos, Lucoryphus and many other Raptors are beginning to mutate.
*More weeks pass, and the situation for the conscripts is truly dire. Katsuhiro is now essentially a hardened veteran and everyone around him is dying of plagues. The sky suddenly parts and an island of floating bones descends from the sky. The palace guns try to shoot it down, but their shots simply pass right through it. The island projects the giant image of Zardu Layak, who declares that the Imperium has abandoned the common folk to die and if the citizens of Terra join Horus now they will be spared. After Layak finishes his speech it begins to rain blood, the ritual to break the Emperor's shield and on Terra and destroy the Aegis has finally begun. Aboard the Nightfall Angron is ejected into space from his maze-prison and immediately makes his way towards the surface.
The Dark Mechanicus reveal their plan constructing massive "Warp-Bane" guns that sap the Aegis of its energy. At the same time massive Towers of Nurgle have been built and are rolling towards the walls, with 3 at Katsuhiro's section alone. Endless swarms of Lost and the Damned assail the last trench network in front of the palace walls. The Aegis finally gives way, and Horus' orbital bombardment sweeps through the Palace defenses devastating everything it comes across. Katsuhiro, running and laughing hysterically at the sheer concept of what hes been through, only survives by running to the hidden tunnel he discovered earlier, where inside he overhears Myzmarada and Doromek planting charges to destroy Bastion 16. Katsuhiro escapes the tunnel just as the Bastion explodes. He emerges witnesses a flaming mass crash to the ground. It's Angron and he begins slaying everything he comes across. Faced with Angron and the incoming Siege Towers Katsuhiro is about to lay down and let himself die when Sanguinius appears and leads a massive Blood Angels charge from the gates. Imperial Fists under Thane as well as Titans from the Legio Solaria come with them. Dorn has relented and allowed the Palace gates be opened and all the surviving Army conscripts be allowed inside as the Space Marines and Titans buy time.
During the battle, the Night Lords begin to land on the wall directly. Raldoran is challenged by Skraivok who is incredibly arrogant. Skraivok is dominating the fighting but suddenly the Daemon in his blade abandons him, leaving his base abilities no match for Raldoran. Skraivok is slashed and thrown off the Wall. Dying on the ground he sees his Daemon Blade's physical form. The Daemon says that he'll save Skraivok's life and give him new power. Skraivok agrees, but the Daemon then adds that to obtain this new power he must endure 6x666 years of torment in the Warp. Skraivok disappears screaming.
Ultimately the Marines/Titans are able to destroy the Nurgle Siege Towers and escort all of the conscripts to safety. Angron charges at the Walls towards Sanguinius and yells a challenge, but the Emperor's remaining barrier still blocks him from getting any closer. Sanguinius salutes Angron with his spear and states that while they'll battle soon, today is not that day.
*Once inside the walls Katsuhiro apologizes to a Marine (who turns out to be Thane) about how hes been a burden and how many astartes died protecting them. Thane states that it doesn't matter who anyone is anymore, mortal and astartes, and the conscripts are all brave warriors for the emperor who should have each others backs. Katsuhiro is attended to by a medic and begins screaming as he sees Doromek smirking at him and disappearing into the crowd. The medic thinks he's delirious and puts him under with drugs.
The final chapter shows Horus again in a comatose state scanning the Warp, Abaddon watching angrily nearby. Layak states that Horus is doing important work, the Siege is being waged in both the Materium and Immaterium and each time Horus enters the Warp he is breaking the Emperor's spirit bit by bit. Layak states that he knows Abaddon's growing resentment against Horus and reveals that Horus is basically now a wildfire. He's glowing bright, swelling with the powers of Chaos but that the Gods are going to see him burn out soon and he will die soon. Abaddon asks what will happen if Horus dies before he kills the Emperor, and Layak states that the Gods will have to find another champion in that case.
FIN