For owners of either Total War: WARHAMMER I or Total War: WARHAMMER II, soon you’ll get Total War: WARHAMMER III - Immortal Empires and this will include The Lost God prologue campaign unlocked as well. When? It’s happening on December 4th, when you should see it pop up in your library.
If you have humble monthly, you already own Warhammer III, as its in this months choice bundle.