This is one of three summaries of the Horus Heresy, a long-running series set in the world of the tabletop wargame Warhammer 40,000. This one’s the long summary (about 4000 words).

There are alternate summaries below:

The Horus Heresy in 100 Words

The Horus Heresy in 500 Words

To learn more about what’s going on, consider reading Summarising The Horus Heresy, or explore the broader set of Horus Heresy essays.

What actually happens in the Horus Heresy? Let’s break it down into three phases.

Phase One: The Heresy Begins

Warmaster Horus Lupercal is one of eighteen primarchs of the Emperor of Mankind. As the favoured son, Horus leads a galactic crusade to reunite the fragmented human Imperium, restoring contact with lost worlds and cleansing aliens, orcs, and the non-compliant. His legion of Space Marines, the Sons of Horus, are joined by Erebus, a commander from the Emperor’s Children legion. During a delicate negotiation with a lost human colony, Erebus secretly steals an ancient blade, causing negotiations to collapse (bk 1, Horus Rising).

Erebus gives the blade to a corrupted planetary governor on the planet Davin. Davin rebels against the Imperium, and Horus and his Sons are sent to re-establish compliance. During the battle, the governor stabs Horus with the ancient blade. Horus falls into a coma and enters the warp, where Erebus tempts him to betray the Emperor and declare for the Chaos Gods. Horus’s sorcerer brother Magnus also appears, trying to dissuade him from that path. Horus chooses betrayal, and the heresy begins (bk 2, False Gods).

Horus quickly corrupts three brother primarchs: Mortarion, Angron, and Fulgrim. Drawing on secretive warrior lodges, Horus purges those unlikely to follow his betrayal. The purge is focalised through Horus’s four lieutenants, the so-called Mournival. Abaddon and Aximand follow Horus into heresy, while Loken and Torgaddon are purged on the planet Isstvan III (bk 3, Galaxy in Flames). Some loyal Space Marines escape the purge, including Nathaniel Garro, a Death Guard under Mortarion, who makes his way back to Terra with warning (bk 4, Flight of the Eisenstein).

We then have a run of books introducing major legions and their primarchs. The first, Fulgrim, follows Fulgrim and his legion (the Emperor’s Children) as they fall to Chaos. The Emperor sends seven legions to attack Horus and Fulgrim and the other traitors at a planet called Isstvan V. In a shocking turn of events, we learn that half of the Emperor’s legions have been secretly turned. Of the seven sent to Isstvan, only three are actually loyal. Those three are almost entirely obliterated in a gruelling sequence known as the Dropsite Massacre. Fulgrim kills the loyalist primarch Ferrus Manus, and, in a moment of despair, is possessed by a daemon (bk 5, Fulgrim).

In a side story, we are introduced to the Dark Angels legion and their primarch Lion El’Jonson. We learn how the primarch landed on their planet, Caliban, and rose to power. We also meet Jonson’s lieutenant Luther. Luther notices an attempt to assassinate the Lion, and, in a moment of jealousy, considers allowing the attempt to succeed. He doesn’t allow it, but as punishment he’s left on Caliban to train up new troops (bk 6, Descent of Angels). Luther will eventually revolt against the Lion in a sequence of events that destroys Caliban, but it doesn’t happen until after the Heresy, so their whole story arc will be left unresolved.

A third book explores the sneaky Alpha Legion, who are contacted by an agent of the Cabal, a shadowy group of aliens trying to defeat Chaos. The Cabal recruit the Alpha Legion not to fight against Horus, but to support him: the Cabal believes that if Horus wins the war, he will self-destruct and obliterate the traitor forces, removing the taint of Chaos from the galaxy. Humanity will die, but the universe will be safe. The Alpha Legion love the idea and hop on board (bk 7, Legion). They immediately start messing with the Raven Guard, survivors of the Drop Site Massacre. The Raven Guard primarch, Corax, had escaped the slaughter with a few of his men. They try to rebuild their shattered legion, but the Alpha Legion intervene and stop them having much success (bk 18, Deliverance Lost).

There are also a bunch of early skirmishes and supporting actions. Traitor battleships start to amass for war on a key loyalist region, Ultramar (governed by primarch Roboute Guilliman and his Ultramarines). Some loyalist Space Marines notice that something’s up and destroy one of the ships before it gets underway (bk 8, Battle for the Abyss). The civil war between traitor and loyalist legions also erupts on Mars, the Imperium’s key manufacturing centre. Traitor factions take control of the planet, but are swiftly blockaded by the Imperium (bk 9, Mechanicum). They lounge around until the invasion of Terra at the end of the series. We also get a medley of various legions and characters in the early stages of the Heresy. There are brutal compliance actions and ignored warnings (bk 10, Tales of Heresy). The Lion rescues some siege engines and gives them to the primarch Perturabo, currying favour for an attempt to become the new Warmaster after Horus is defeated. Perturabo, of course, is secretly a traitor, meaning the dumbass Lion has accidentally supported the rebellion (bk 11, Fallen Angels). Meanwhile, agents of Horus and the Emperor try to assassinate their respective enemy leaders; when neither plot is successful, Horus and the Emperor step in and codify their assassin orders (bk 13, Nemesis).

We also see the fallout of Magnus’s attempt to save Horus from Chaos. Magnus is a sorcerer, using psychic powers banned by the Emperor. We see from his perspective the attempt to stop Horus falling to Chaos. When that fails, he tries to psychically project himself to the Emperor and pass on a warning. He blunders into a very delicate experiment, ruining it, killing thousands, and accidentally trapping the Emperor underground. The Emperor, suitably annoyed, sends the primarch Leman Russ and his Space Wolves to chastise Magnus. Russ destroys Magnus’s physical body and razes his home planet of Prospero. Magnus survives as a psychic force and teleports into hiding with the remnants of his legion (bk 12, A Thousand Sons). In a paired book told from the Space Wolves perspective, we additionally learn that the Chaos Gods have engineered that conflict to weaken the Space Wolves and remove Magnus and the Thousand Sons as potential loyalist allies (bk 15, Prospero Burns). In a third book, a psyker on Terra receives a vision in the wake of Magnus’s disastrous projection. He passes that vision on to the Emperor, who sees and accepts how the Heresy will end (bk 17, The Outcast Dead).

After those opening movements, we start to ramp up to the Heresy’s second phase: the war in Ultramar. We are formally introduced to Lorgar, primarch of the Word Bearers. Before the Heresy, Lorgar was punished for worshipping the Emperor as a god. He discovered the Chaos Gods and sulkily pledged his loyalty to them instead. He turned some of his Space Marines into demon-possessed monsters and used the warrior lodges to begin the process of infiltrating and corrupting other legions (bk 14, The First Heretic). What a bitch. There are some other little plot ornaments in two short story collections: we see some of the other traitor legions purging (or attempting to purge) their loyalist factions (bk 16, Age of Darkness), and it turns out that Fulgrim has regained control of his body from the demon (bk 20, The Primarchs).

Phase Two: The Ultramar Campaign

The second phase opens in full with a genuinely exceptional novel: Know No Fear. The Word Bearers make a surprise attack on Calth, a key shipyard world in Ultramar. They don’t quite manage to murder Roboute Guilliman, the Ultramarine primarch, but they do create a massive psychic storm, the Ruinstorm, which splits the galaxy in half and makes it difficult for loyalist forces to move around (bk 19, Know No Fear). They also poison the sun and make Calth’s surface radioactive. Ultramarine and traitor forces retreat into caverns underground and continue to fight (bk 25, Mark of Calth).

We’re also formally introduced to Sanguinius, primarch of the Blood Angels, who Chaos forces lure into a trap on a planet called Signis Prime. Chaos turns the Blood Angels into blood-hungry vampire monsters and try to get Sanguinius to sacrifice himself to save his legion. The plan fails, but the legion is cursed with sick vampire powers (bk 21, Fear to Tread). Elsewhere, traitor primarchs Fulgrim and Angron both ascend, becoming Daemon Princes against the backdrop of the Ultramar campaign (bk 23, Angel Exterminatus, and bk 24, Betrayer).

Vulkan, the third loyalist primarch ambushed at Isstvan, also turns out to be alive. He’s been captured by Konrad Curze, primarch of the Night Haunters, who repeatedly tortures and murders him. Vulkan, apparently, is immortal, or more specifically comes back to life after he’s killed – which suits Curze, who’s a creepy torture freak. Vulkan hits him with a hammer and escapes by teleporting into the vacuum of space (bk 26, Vulkan Lives). He dies, obviously, because it’s vacuum, and his body falls to the surface of the planet Macragge, which is in the Ultramar sector. A whole bunch of characters then all turn up at once. Guilliman lives on Macragge, so he’s already there. The Lion turns up with Curze, who’s running loose on one of his ships (due to some nonsense in the short story collection Shadows of Treachery, bk 22, don’t worry about it). Sanguinius also stumbles in, fresh from the trap at Signis Prime. Curze immediately escapes onto Macragge and starts causing trouble. Vulkan comes back to life, but this time he’s insane. A Cabal agent from Legion turns up and stabs Vulkan with a magic spear, and Vulkan dies again – and then weirdly doesn’t come back to life. The three loyalist primarchs believe that Terra might be lost, and declare Macragge the centre of a new Imperium, with Sanguinius as its Emperor (bk 27, The Unremembered Empire). It’s a very busy book.

Meanwhile, we’re introduced to the White Scars, another loyalist legion. Twenty-eight books and we’re still introducing new legions. Their primarch, Jaghatai Khan, is a bit of a wild child. He runs free. Mortarion tries to convert the Khan to Chaos and fails. The White Scars have a brief, violent conflict over whether they should betray the Emperor, and the loyalist side wins (bk 28, Scars).

Horus, who’s been sitting around plotting this whole time, hauls himself up and over to Molech, where he finds a gate into the warp. He enters and wins the loyalty of a demon army. Also, his Mournival lieutenant Loken, who was betrayed and left for dead at Isstvan III, turns out to be alive. He sneaks onto Horus’s battleship, where there’s a short confrontation and a lucky escape (bk 29, Vengeful Spirit).

There’s a bunch of further ornament with little narrative consequence – mostly just various minor manoeuvres and short character vignettes. In one story, Horus monologues to the skull of Ferrus Manus in an overwrought Hamlet reference (bk 31, Legacies of Betrayal). Another deals with the White Scars put on trial after the short civil war in Scars (bk 33, War Without End), and another flashes back to before the Heresy, where the Emperor and Horus fight some orcs (bk 35, Eye of Terra). The Silent War is the only collection with an overt theme, centring around the various machinations of the agents of Malcador the Sigillite, the Emperor’s closest advisor (bk 37).

A few other, scattered things happen as well. Some Space Marines who escaped Isstvan find a jungle planet filled with dinosaurs. They try to protect some human colonists, but it turns out the colonists are actually evil cultists. Everyone dies, and some demons fly off on a spaceship (bk 30, The Damnation of Pythos). On Macragge, survivors from Vulkan’s legion (the Salamanders) escort Vulkan’s body back to their homeworld Nocturne, where he magically comes back to life (bk 32, Deathfire).

Meanwhile, the Second Imperium has been making use of the Pharos, an alien beacon that helps with warp travel during the Ruinstorm. Some of Curze’s Night Haunters find it and torture a bunch of mechanics before Guilliman arrives and saves them (bk 34, Pharos). The triumvirate of Guilliman, Sanguinius, and the Lion are finally able to capture Curze, and Curze narks on the Lion, who while chasing Curze had bombed a planet from orbit, even though he’d promised not to. The Lion gets banished for being a dickhead, ending the rule of the triumvirate, and Ultramar is finally safe (bk 38, Angels of Caliban). Also, Curze has this whole thing where he tells the future, and he claims he’s going to be killed by an assassin from the Emperor. This suggests to the others that a) Terra still exists and b) the Emperor’s still alive. They’d better get a move on.

Phase Three: The Invasion of Terra

Everyone starts to look towards Terra. The Ruinstorm has made warp travel difficult, but there have been some various little developments and sneaky tricks that might allow the loyalists to travel. The White Scars find an ancient relic which lets them travel back to Terra (bk 36, The Path of Heaven). Vulkan and a handful of Salamanders make their way as well (bk 47, Old Earth). The three loyalist legions from Ultramar, Guilliman’s Ultramarines, the Lion’s Dark Angels, and Sanguinius’s Blood Angels, go via Davin, where Horus was originally corrupted. It was the start of the Heresy, and it’s also where the Ruinstorm is anchored. The loyalists blow up Davin and fight off the demons from The Damnation of Pythos, who turn up in their spaceship. They also shoot Curze into space in a coffin. The Ruinstorm starts to settle, and the way to Terra becomes relatively clear (bk 46, Ruinstorm). There are a bunch of rearguard fleets protecting the traitors, so the Lion and Guilliman plan to run interference while Sanguinius rushes through to defend Terra. The Lion and Guilliman essentially become the third act’s jeopardy. If Horus wants to capture Terra, he has to do it before the Lion and Guilliman arrive with their massive fleet.

On Terra itself, there’s an early attack. Rogal Dorn, who’s been fortifying the planet for nearly forty books, suffers an embarrassing invasion by the Alpha Legion. He tracks down Alpharius, leader of the Alpha Legion, who prances about telling Dorn that it’s actually a super clever double triple cross. Dorn cuts his hands off and murks him (bk 39, Praetorian of Dorn). Garro is also hanging around – he brought the warning about the Heresy in Flight of the Eisenstein, and then got recruited to start the Grey Knights, a secret psychic Space Marine group who fight Chaos (bk 42, Garro).

We also learn what the Emperor’s been doing this whole time. Turns out he had a secret plan for a different type of space travel – abandoning the warp, where the Chaos Gods live, and moving instead through the webway, which is really just a fancy warp alternative that the alien Eldar used to use. When Magnus astral projected into Terra, back at the start of the series, he ruined a delicate webway machine. A bunch of Chaos daemons started pouring in, and the Emperor’s been busy dealing with that (bk 41, The Master of Mankind). It’s going poorly.

Elsewhere, various plots are resolved. Corax goofs around with the Raptors, a bunch of soldiers intended to be Space Marines but turned into mutant hybrids by Alpha Legion interference. He raids traitor supply lines and gets a bit down about the war (bk 40, Corax). Shadrak Meduson, a survivor from Isstvan V, is left to die by his fellow Iron Hands. Meduson is a legend. He spends the whole series leading hit and runs on the traitors, and very nearly assassinates Horus twice. Eventually he gets drawn into a game of cat-and-mouse with a Sons of Horus captain, Tybalt Marr, who catches him. Meduson is abandoned by the rest of his fractured legion, and Marr chops his head off (bk 43, Shattered Legions). Rest proud, buddy. There’s also a bunch of inconsequential guff, where the writers anthologise a bunch of shorter works – mostly just to get them into the canon before the series ends (bk 48, The Burden of Loyalty; bk 50, Born of Flame; bk 52, Heralds of the Siege). Some of this material includes stories published in previous Horus Heresy anthologies (the short story ‘Artefacts’ was published in bk 33, War Without End, and again in bk 50, Born of Flame), which really makes you feel like they’re wasting your time.

The traitor primarchs finally all get around to the war against Terra. Magnus has a fractured personality after astral projecting to Terra and getting beaten up by Leman Russ. One of his Thousand Sons, Ahriman, goes on a quest to find all the missing pieces, fragments of Magnus acting as independent living beings. He finds most of them. The last one is stuck on Terra, probably still with the Emperor, so Magnus signs up to the invasion to get it back (bk 44, The Crimson King). Angron’s a slavering nut job at this point, and Fulgrim is off getting freaky with a snake daemon, so Perturabo and Lorgar corral them both towards Terra (bk 51, Slaves to Darkness). The traitors also kick the shit out of some planets along the way. There’s a tank battle on Tallarn (bk 45, Tallarn), and a massive battle in the Beta-Garmon system between Titans, essentially 40K’s giant mecha-robots (bk 53, Titandeath). Leman Russ also ducks in and stabs Horus with a magic spear, weakening the control of Chaos. It won’t swing the balance, but it will let Horus return to himself in the moments before his death (bk 49, Wolfsbane). Mortarion finally commits himself to the plague god Nurgle, in the final title before the Siege of Terra (bk 54, The Buried Dagger), and we’re into the end of the heresy.

Phase Three, continued: The Siege of Terra

The traitor forces enter the solar system, and there’s a massive space battle. While the invasion falters around the outer planets, Horus uses some pre-planned sorcery on Halley’s Comet to skip right into the centre of the system. Rogal Dorn is almost killed by a swarm of daemons, introduced accidentally onto his ship by the human Mersadie Oliton, who sacrifices herself to stop them all (bk 55, The Solar War).

Although Horus has the firepower to destroy Terra from orbit, he needs to conquer the Emperor’s palace and defeat him in person – the warp values symbolism over pragmatism. Horus’s forces start to land on Terra around the Emperor’s palace. A shield protects the city from bombardment and holds back daemons and warp-infested beings like the traitor primarchs, so the traitors have to breach the walls on foot. Conscripted soldiers hold the trenches outside the walls against beastmen, plagues, and constant bombing. Some of them manage to survive, and that’s about as good as things get (bk 56, The Lost and the Damned).

Perturabo manages the siege strategy on behalf of the traitor primarchs, who are otherwise consumed by their various Chaos Gods. He charges his lieutenant Kroeger with breaching the Lion’s Gate spaceport, so that the traitors can start bringing in Titan war machines. Kroeger is a crude, thuggish brawler, but through bloody-mindedness and sheer overwhelming numbers, he takes the port. The incipient cult of the God-Emperor also serves as inadvertent entry point for various daemons and plague monsters (bk 57, The First Wall).

Horus’s lieutenant, Abaddon, then leads a sneak attack, drilling under the Saturnine Wall. The loyalists spot him coming and set a trap, butchering the invading traitors, including many of Horus’s chief lieutenants, who are slain by Loken, the surviving loyalist from Horus’s legion. Fulgrim climbs up on top of the Wall, and Rogal Dorn fights him off. Fulgrim and the Emperor’s Children quit the war in a huff (bk 58, Saturnine).

The traitors start landing Titan mechs at captured spaceports. Horus replaces Perturabo with Mortarion as chief siege architect: Perturabo quits the war, also in a huff, and Mortarion spreads a bunch of plague. The Titans rock up to the next line of battlements and blow their way in (bk 59, Mortis). Jaghatai and the White Scars then ball out and attack Mortarion in a desperate last-ditch assault. Jaghatai dies (or ends up in a death-coma), but banishes Mortarion (bk 60, Warhawk).

The remaining primarchs start to square off. Magnus is attacking from under the Palace, through the webway: Vulkan finds him and banishes him. Angron and the World Eaters get blocked by Sanguinius, who banishes Angron and then the Red Angel, an evil daemon that was wrapped up in the Blood Angel curse back in Fear to Tread (bk 61, Echoes of Eternity).

The loyalists have now seen off most of the traitor primarchs, but there are so many assorted traitor forces that they’re still going to lose. Sanguinius’s battle corresponds with closing the Eternity Gate, the last gate leading into the heart of the Palace itself. The siege now enters its final stage. The Emperor rises from his throne, where he’s been maintaining the psychic shields to hold off a massive daemon invasion through the basement. His advisor, Malcador the Sigillite, takes his place. Horus lowers the shields on his flagship, the Vengeful Spirit, and the Emperor jumps in with four strike teams: one for him, and three more with Rogal Dorn, Sanguinius, and Valdor (the head of his Custodian Guard). It’s very much a trap. Each group is separated and attacked (bk 62, The End and the Death vol I).

To combat Horus, the Emperor starts to gather power from the warp and transforms into a giant evil orb of death. That’s secretly what Chaos have wanted all along: they want the Emperor to absorb the warp and turn into a new, fifth God of Chaos. Other Chaos Gods have been born before, so they’re hoping to turn the Emperor into the Dark King, the god of ruin. The Emperor is confronted by Oll Persson, an immortal human who’s been hopping about through the series: Oll persuades the Emperor to relinquish his power, even though he won’t be able to defeat Horus in his mere human form. Sanguinius fights his way to Horus and gets fucking owned (bk 63, The End and the Death vol II).

The Emperor finally makes it to Horus and gets bounced around like a beach ball. Horus is much too powerful. At the same time, a bunch of human refugees hide in the Hollow Mountain, where the Astronomicon is stored – essentially a psychic beacon that guides ships through the warp. It’s not working, but the humans all proclaim their faith in the Emperor very loudly, and that psychic force gives the Emperor enough strength for a final bout with Horus. He stabs Horus with a magic knife that he got from Oll (long story, doesn’t matter), and Horus finally and absolutely dies. The Emperor’s broken body is installed back on the Throne, where it is held in stasis on the edge of life and death. The traitors are sundered, Guilliman arrives with the relief forces, and the dust settles on a crippled Imperium (bk 64, The End and the Death vol III).