|Commanders1 = <big>[[File:Raptor Imperialis Remake Clear V2.png |20px]] [[Emperor of Mankind]] (WIA)</big><br>

[[File:IF Icon.png|20px]] [[Rogal Dorn]]<br>

[[File:WS Icon.png|20px]] [[Jaghatai Khan]] (WIA)<br>

[[File:SAL Icon.png|20px]] [[Vulkan]] (Hidden){{Fn|14}}<br>

[[File:IF Icon.png|20px]] [[Kestros|Archamus]]{{Fn|13d}}<br>

[[File:IF Icon.png|20px]] [[Fafnir Rann]]{{Fn|10j}}<br>

[[File:IF Icon.png|20px]] [[Maximus Thane]]{{Fn|8}}<br>

[[File:IF Icon.png|20px]] [[Fisk Halen]]{{Fn|13a}}<br>

[[File:IF Icon.png|20px]] [[Sevastin Haeger]]{{Fn|10j}}<br>

[[File:BA Icon.png|20px]] [[Raldoron]]<br>

[[File:BA Icon.png|20px]] [[Azkaellon]]<br>

[[File:BA Icon.png|20px]] [[Nassir Amit]]<br>

[[File:BA Icon.png|20px]] [[Zephon (Blood Angels)|Zephon]]<br>

[[File:BA Icon.png|20px]] [[Anzarael]]<br>

[[File:BA Icon.png|20px]] [[Khoradal Furio]]<br>

[[File:BA Icon.png|20px]] [[Satel Aimery]]<br>

[[File:BA Icon.png|20px]] [[Emhon Lux]] (WIA)<br>

[[File:WS Icon.png|20px]] [[Ganzorig Noyan-Khan]]<br>

[[File:WS Icon.png|20px]] [[Shiban Khan]]{{Fn|13g}}<br>

[[File:WS Icon.png|20px]] [[Jangsai Khan]]{{Fn|13g}}<br>

[[File:WS Icon.png|20px]] [[Namahi]]<br>

[[File:DA Icon.png|20px]] [[Corswain]]{{Fn|13g}}<br>

[[File:SAL Icon.png|20px]] [[Atok Abidemi]]{{Fn|14}}<br>

[[File:SAL Icon.png|20px]] [[Igen Gargo]]{{Fn|14}}<br>

[[File:Custode Icon.png|20px]] [[Amon Tauromachian]]{{Fn|10j}}<br>

[[File:Machina Opus.png|20px]] [[Zagreus Kane]]<br>

[[File:Solaria Icon.png|20px]] [[Esha Ani Mohana]]{{Fn|9m}}<br>

[[File:Sinister Icon.png|20px]] [[Aurum (Prefect)|Aurum]]<br>

[[File:Imperial Army Symbol.jpg|20px]] [[Adreen]]<br>

[[File:Imperial Army Symbol.jpg|20px]] [[Ilya Ravallion]]<br>

[[File:Imperial Army Symbol.jpg|20px]] [[Sandrine Icaro]]<br>

[[File:Imperial Army Symbol.jpg|20px]] [[Aldana Agathe]]{{Fn|13}}<br>

[[File:Imperial Army Symbol.jpg|20px]] [[Clement Brohn]]<br>

[[File:Custode Icon.png|20px]] [[Diocletian Coros]]<br>

[[File:IW Icon.png|20px]] [[Perturabo]] (AWOL)<br>

[[File:WE Icon.png|20px]] [[Angron]] (Banished)<br>

[[File:DM Icon.png|20px]] [[Kelbor-Hal]]<br>

[[File:SOH icon.png|20px]] [[Ezekyle Abaddon]]<br>

[[File:SOH icon.png|20px]] [[Falkus Kibre]] (KIA?){{Fn|13f}}<br>

[[File:SOH icon.png|20px]] [[Lev Goshen]] (MIA){{Fn|13f}}<br>

[[File:SOH icon.png|20px]] [[Azelas Baraxa]]<br>

[[File:SOH icon.png|20px]] [[Xhofar Beruddin]]<br>

[[File:SOH icon.png|20px]] [[Malabreux]]<br>

[[File:SOH icon.png|20px]] [[Hellas Sycar]]<br>

[[File:SOH icon.png|20px]] [[Taras Balt]]<br>

[[File:SOH icon.png|20px]] [[Vorus Ikari]]<br>

[[File:SOH icon.png|20px]] [[Kalintus]]<br>

[[File:SOH icon.png|20px]] [[Zistrion]]<br>

[[File:SOH icon.png|20px]] [[Ekron Fal]]<br>

[[File:WB Icon.png|20px]] [[Erebus]]<br>

[[File:DG Icon.png|20px]] [[Typhus]]<br>

[[File:DG Icon.png|20px]] [[Caipha Morarg]]<br>

[[File:DG Icon.png|20px]] [[Zadal Crosius]]<br>

[[File:WE Icon.png|20px]] [[Khârn]] (KIA)<br>

[[File:WE Icon.png|20px]] [[Shâhka Bloodless|Shâhka]]{{Fn|23}}<br>

[[File:IW Icon.png|20px]] [[Kroeger]] (AWOL){{Fn|10j}}<br>

[[File:IW Icon.png|20px]] [[Barban Falk]] (AWOL){{Fn|10j}}<br>

[[File:NL Icon.png|20px]] [[Gendor Skraivok]] (MIA)<br>

[[File:NL Icon.png|20px]] [[Krostovok]]<br>

On the thirteenth day of Secundus, the bombardment of Terra began. From orbit, Horus' ships laid down an unrelenting barrage of missiles and energy beams. The first shell targeted the [[Sanctum Imperialis]], the Emperor's personal quarters, for symbolic reasons, though it did no damage and was quickly intercepted by many point defense systems. A vicious exchange of fire took place as the massive array of anti-ship gun batteries on Terra opened up on the traitor ships. Losses were incurred by both sides, but the traitor fleet in particular took heavy losses in space but endured through sheer weight of numbers.{{Fn|9a}} Horus' ultimate objective was the [[Imperial Palace]], seat of the Emperor himself. The orbital bombardment was some of the most intense ever witnessed and shook even the courageous and battle-hardened loyalists [[Space Marines]].{{Fn|1b}} However damage to the actual Palace itself was minimal due to a sophisticated multi-layered and self-repairing [[Void Shield]] system known as '''The Aegis''' as well as the fact that Terra's last orbital plate was placed over the Palace to serve as a shield and gun platform. However, due to a reactor flaw in one of the Aegis' reactors, a small weakness appeared in the Palace network centered around a Bastion outside the Helios Gate.{{Fn|9a}} Inside the depths of the Palace itself, the Emperor, attempting to keep the [[Golden Throne]] under control, could only brood over the desperate events occurring far above.{{Fn|1b}}

The bombardment continued for days as [[Perturabo]] probed for weaknesses in the Aegis, discovering the flaw around Bastion-16. The Aegis around this area was weakened enough to allow for slow-moving aircraft to penetrate. Despite the ragings of [[Angron]] and the boredom of [[Fulgrim]], Horus would not launch an all-out attack yet. The Emperor's might not only prevented [[Daemon]]s from manifesting on Terra, but also threatened to destroy the [[Daemon Primarch]]'s should they set foot upon its soil. [[Magnus]] and [[Zardu Layak]] surmised a ritual to eliminate the Emperor's barrier by spilling enough blood across Terra. Horus began to increasingly spend his time comatose, his spirit inside the Warp searching for lore as well as weakening the Emperor's spirit,{{Fn|9b}} eventually designating [[First Captain]] [[Ezekyle Abaddon]] as his proxy.{{Fn|9d}} After 12 days the bombardments continued, but were matched with the first ground assaults. Thousands of traitor vessels crashed into Terra's surface. Many were destroyed by the Palace defenses, but many more landed and unleashed endless hordes of Traitor [[Imperial Army]], [[Beastmen]], [[Cultist]]s, and [[Mutant]]s. These were largely distractionary measures to support the traitors' true objectives as they ravaged the planet, as well as support the traitor aerial attacks on the Palace's anti-ship guns and void shield generators. The loyalists responded with their own aerial defenses, and massive dogfights erupted over the Palace as the conscripts outermost trenches around the walls held against the attacks by the [[Lost and the Damned]]. While the Palace held, elsewhere across Terra major population centers fell such as Lundun, Noy Zaylant Hive, Neork, and Braslya.{{Fn|9l}}

**''[[Throne of the Underworld]]''

[[Sanguinius]]' stand at the [[Eternity Gate]] is stated to have been roughly 200 days into the battle.{{Fn|22h}} Mere hours are then depicted as having passed from the closing of the Eternity Gate until the time when the ''[[Vengeful Spirit]]'' drops its [[Void Shield]]s, prompting the [[Emperor]] to lead the final assault.{{Fn|24b}} Time essentially freezes by this point due to the [[Warp]]'s presence on Terra, with those involved in the battle experiencing wildly different timeframes for the remainder of the siege.{{Fn|26b}}

There are many differences with the narrative of the Siege of Terra as originally told in older sources with that of the [[Black Library (BL Publishing)|Black Library]] novel series. Most notably these include:

*In the original lore, [[Lorgar]] himself led the Word Bearers on Terra.{{Fn|1a}} However as of the events of the novel [[Slaves to Darkness (Novel)|Slaves to Darkness]], Lorgar is banished from Horus' sight for attempting to usurp him and instead the Word Bearers at Terra were led by [[Zardu Layak]].{{Fn|4}}

*The [[Sky Fortress]] was used for the defense of the Eternity Gate, not for the retaking of the Lion's Gate Spaceport by Jaghatai Khan.{{Fn|1d}}

Two versions of the Emperor's final words prior/during his interment upon the Golden Throne are given.

*From [[Horus Heresy: Collected Visions]]:

**{{Endn|10j}}: Dramatis Personae

*{{Endn|11}}: [[Morningstar (Audio Drama)]]

*{{Endn|13}}: [[Saturnine (Novel)]] Dramatis Personae

**{{Endn|13a}}: Part 1, Chapter 2-3

**{{Endn|20f}}: Aftermath

**{{Endn|20g}}: Cover

*{{Endn|22}}: [[Echoes of Eternity (Novel)]] - Dramatis Personae

**{{Endn|22a}}: Chapter 3

**{{Endn|24q}}: Dramatis Personae

**{{Endn|24r}}: 1:xvii

*26: [[The End and the Death: Volume II (Novel)]]:

**{{Endn|26a}}: 7:xxvi

**{{Endn|27l}}: 9:vi

**{{Endn|27m}}: Interludes: Distant Sons - i

*{{Endn|29}}: [[System Purge (Short Story)]] - [[Era of Ruin (Anthology)]]

*{{Endn|30}}: [[The Horus Heresy: Age of Darkness 3rd Edition Rulebook]], pgs. 144-149

**{{Endn|31a}}: Chapter 2

**{{Endn|31b}}: Chapter 9

**{{Endn|31e}}: Chapter 7

**{{Endn|31f}}: Chapter 11

{{HHBattles}}