Spoilers: Archaon’s allegiance and rise to the title of Everchosen; no novel ending or quest-battle outcome.

A servant of Sigmar turns north
Archaon began as a Sigmarite Templar. The discovery of an ancient religious doctrine broke his allegiance to Sigmar and drove him north into the Chaos Wastes, where he offered himself to the Chaos Gods.
Their blessing set him on the path to becoming the Everchosen. His former faith makes the change of allegiance central to his identity: a warrior once devoted to Sigmar becomes a champion of the powers that threaten the mortal world.
A champion of the rival gods
The title Everchosen marks Archaon as a champion favoured by the Chaos Gods together. His purpose is to unite their followers and bring ruin to the world. That shared allegiance places him above a warband devoted to a single patron.
His veteran Chaos Knights are known as the Swords of Chaos, and he rides the daemonic steed Dorghar. These companions belong to his identity as a war leader; his Immortal Empires campaign lets the player decide how that ambition unfolds.
Two novels about the man before the title
Rob Sanders's Archaon: Everchosen is book 1 of the Archaon duology, filed with The End Times. The jacket starts from the warlord poised over the world and then asks for the earlier man: a devout servant of Sigmar, a knight of the Empire, and the events that could put him on the path to the Ruinous Powers. The edition includes the short story Archaon: The Fall and the Rise. The jacket asks those questions and leaves the answers in the book.
Book 2, Archaon: Lord of Chaos, continues after the fall. The former templar seeks six great treasures of Chaos and means to bring death to the world as Everchosen. The journey the jacket names runs from the southern Chaos Wastes to Bretonnia, through perils and betrayals. Be'lakor, the first Daemon Prince, plots to destroy the would-be Everchosen and take the mantle himself. That rivalry is the novel's premise. Whether either of them succeeds is the book's business.
The campaign choice is not the duology
Update 9.0 gives the playable Archaon two systems that use End Times language. After a victory over another Chaos legendary lord he can subjugate them. Warriors of Chaos victims are confederated and their settlements pass to him. Lords of Khorne, Nurgle, Slaanesh, and Tzeentch join his character pool, and their regions pass to a new vassal of that god. The list includes Kholek, Sigvald, Azazel, Festus, Valkia, Vilitch, Be'lakor, and the Glottkin, plus the mono-god lords. It excludes Beastmen, Chaos Dwarfs, Daemons of Chaos, and Norsca. A wiped Chaos lord can also return later as an army to fight, and winning that fight adds them.
The other system is a choice at the short victory: keep conquering, or become Lord of the End Times. That second option blocks Archaon and his vassals from taking further settlements. The campaign turns to destruction, razing, and devastation. It is a rule on the Warhost of the Apocalypse. It is not a summary of either novel, and a sandbox that never clicks it has not contradicted the books.
Age of Sigmar, labeled as itself
Archaon continues in Warhammer Age of Sigmar as the Everchosen of the Mortal Realms. Robbie MacNiven's Scourge of Fate is one verified novel on that side of the line. Its viewpoint is Vanik the Black Pilgrim, a warrior trying to rise into the Fifth Circle of the Varanguard, Archaon's elite. A betrayal hits the Varanspire, Archaon's fortress, and Vanik's hunt is no longer only the hero of Order he was sent to kill. The jacket calls Archaon the most powerful of the Dark Gods' champions and the Three-Eyed King.
That is Mortal Realms fiction. It is not a prologue to Immortal Empires, and the Varanguard are not the Swords of Chaos on the Old World campaign map. The Old World duology and the game's razing button stay in their own sections.
Book 5 of The End Times
The Lord of the End Times is Josh Reynolds’s fifth numbered novel of The End Times, sold as an audiobook narrated by Andrew James Spooner. The Doom of the Old World omnibus, which reprints this novel, says Archaon prepares his war host to sack Middenheim, the last bulwark named in that jacket. The standalone listening page is the edition linked below. Nagash is the other figure that edition places against the march. The two Archaon novels already on this page are the man before the title. Book 5 is the later war. A sandbox that never clicks Lord of the End Times has not replayed it.
Archaon the Everchosen: gameplay and availability · More character stories
Sources
- Archaon: Everchosen — Black Library — blacklibrary.com
- Archaon: Lord of Chaos — Black Library — blacklibrary.com
- Scourge of Fate — Black Library — blacklibrary.com
- Update 9.0 release notes — Total War — totalwar.com
- Update 9.0 release notes — community.creative-assembly.com
- Patch 9.0.5 notes — store.steampowered.com
- Immortal Empires starting positions — totalwar.com
- Chaos Warriors Race Pack — store.steampowered.com
- SEGA Immortal Empires ownership guide — support.sega.com