Total War and Creative Assembly are giving away a big gift to fans at the start of next month, as Total War: Warhammer and Warhammer II owners will get access to the Immortal Empires campaign from Warhammer III, as well as its Lost God prologue.
As explained in a post on Steam, you don't need to do anything to see this gift dropped into your library, and owners of the previous games on PC will just have Total War: Warhammer III added to their libraries from the 4th of December. Immortal Empires is basically the main way a lot of people play Total War: Warhammer III, as it gives players access to the entire map, in which they can fight every faction introduced so far, as well as build sweeping empires.
If you do own Total War: Warhammer III, nothing is going to change for you. If you don't, then the campaign and prologue is all you'll get access to. You won't get the base game's initial story campaign Realm of Chaos, nor will you be able to play with the legendary lords included in Warhammer III unless you pay for an upgrade pack. No matter which version of the game you own, though, you can buy DLC for Immortal Empires to add more campaigns.