Spoilers: The partnership as the game presents it. No campaign ending.

Who she is in Total War
Isabella and Vlad are the two ways into Sylvania. Choose her and she is the lord. Vlad starts as the hero beside her. Update 9.0 adds a skill that can later make that hero a lord of his own. The decision is whether the pair stay in one army or cover two borders. The shared Raise Dead economy still has to feed both.
She mixes vampiric fighting with spells. A supported Vlad can hold one fight while she shapes another. Skeleton Warriors keep the front joined. Grave Guard take the contact you cannot afford to lose. Sending both characters after one fleeing target leaves the line without them.
Sylvania, Vlad, and the aristocracy
Update 3, the free-LC note that returned Vlad to the first game, calls him the first and arguably the greatest of the Vampire Counts, and says he tainted the human aristocracy of Sylvania and forged a kingdom of the undead in the heart of the Empire. The Isabella lord page then places her in that same nobility, beside him. The campaign select screen is where that partnership becomes a starting army.
Mannfred, in that same Update 3 note, is not best pleased and agrees to a suspension of hostilities while the new Emperor and the power in the north both threaten. That is the first game’s framing of three vampires who can share a race and still open different campaigns. It is not a script for who must win Sylvania.
The court around the pair
Walach Harkon and Dieter Helsnicht can join through the paid Nagash content after their unlocks. Walach is a combat caster. Dieter is an offensive caster. They are optional tools. The Sylvania start does not require them. Ulrika’s technology route belongs to Neferata’s faction, not to this one.
Inheritance, Dominion, and Retribution are the von Carstein novels. Vlad is the protagonist of the first. Mannfred is the protagonist of the third. Isabella’s campaign stays on the lord select and on the partnership the game gives her. The books are linked from Vlad’s page and from Mannfred’s.
Isabella von Carstein: gameplay and availability · More character stories
Sources
- Total War: Warhammer Update 3 — Total War Wiki — wiki.totalwar.com
- Update 9.0 release notes — Total War — totalwar.com
- Immortal Empires DLC Ownership Guide — SEGA — support.sega.com
- Immortal Empires DLC Ownership Guide — SEGA — support.sega.com
- Update 9.0 release notes — CA — community.creative-assembly.com