Spoilers: The Runefang wound and the Middenland opening. No duel result and no campaign ending.

Who he is in Total War
Boris leads Middenland. Boris Todbringer – Lords of the End Times unlocks him and Ar-Ulric Emil Valgeir. The pack also grants Immortal Empires access on its own. His opening is in Middenland, with Weismund held by Clan Gnaw. The Great Temple of Ulric in Middenheim is the faction's centre. Defeating enemies earns Fervour for the Temple's Sanctuaries, and the Flame of Ulric supports further benefits.
He is a melee Elector. Teutogen Guard and Knights of the White Wolf push the army into close combat, and the wider Empire roster still offers guns. Emil Valgeir is the Ulrican legendary hero. Volkmar's Cult of Sigmar is a different faith and a different faction. Franz's Reikland campaign is the imperial office Boris once contested.
The eye and the forest
Creative Assembly's 2016 Beastmen introduction calls Boris the Graf of Middenheim and the only lord of men to have bested Khazrak. Boris deprived him of an eye with a slash of his Runefang. Khazrak then stalks that nemesis, meaning to answer in kind. The article's story campaign, An Eye For An Eye, is a bespoke forest map from the Reikland Channel to Hochland. That map is not the Immortal Empires start, and the eye is background, not a wound dealt in a player's later battle.
The same article places Khazrak's older grand-campaign start at Tobaro. Boris's Middenland opening, with Clan Gnaw in Weismund, is the later Empire campaign. Reports of Beastmen on that campaign are decisions about where to march. They do not replay the 2016 story on a new map.
Elector, then emperor's rival, then ally of the realm
Boris once contested Karl Franz for the imperial title. He remains Elector Count of Middenland and a defender of the Empire. Two leaders inside one realm are not one faction. Franz spends Imperial Authority on the Electors. Boris spends Fervour on Ulric's temple. The contest for the throne is earlier than either sandbox.
Warhammer Community's Old World almanack sets an earlier Empire still in the Anarchy, after the Era of the Three Emperors, with Electors feuding and the Elector of Middenheim among the claimants. That tabletop present is not Boris's Immortal Empires reign beside an emperor who already holds the throne. The eye, the temple, and the election sit in the later Fantasy presentation the game uses.
Boris Todbringer: gameplay and availability · More character stories
Sources
- Immortal Empires DLC Ownership Guide — SEGA — support.sega.com
- The Beastmen — Total War — totalwar.com
- Lords of the End Times DLC overview — Total War — totalwar.com
- Lords of the End Times: Meet Boris Todbringer — Creative Assembly — community.creative-assembly.com
- Boris Todbringer – Lords of the End Times — Total War on Steam — store.steampowered.com
- Boris's Lord Select Screen & Starting Position — Creative Assembly — community.creative-assembly.com