This is the fifteenth game in the Total War series and the third game in the Warhammer subseries, following on from Total War: Warhammer (2016) and the unsurprisingly-named Total War: Warhammer II (2017). The long gap in release was due to the studio refocusing on their historical games for several years, resulting in Thrones of Britannia: A Total War Saga (2018), Total War: Three Kingdoms (2019) and Troy: A Total War Saga (2020).
Warhammer III focuses on the icy arctic wastes, featuring four Chaos factions (one each for the minions of Khorne, Nurgle, Slaanesh and Tzeentch), the noble kingdom of Kislev and at least part of the great Empire of Grand Cathay. These faction can engage in battle with one another for supremacy for their part of the Old World. Additional factions will, inevitably, be added in post-release DLC.
Warhammer III can also merge its campaign map with that of Warhammer I and II in the Mortal Empires campaign, which will now become a near globe-spanning megagame including all or most of the factions from the first two games.
A release date has not yet been set apart from "late" 2021, but the game will launch on PC simultaneously on Steam and Epic Game Store (no split release strategy as with Troy). CA do advise that at the moment it will only be possible to merge the three games into one on the same platform, so owners will need all three games on either Steam or Epic, not split between them.
They may be release synergy (gags) between Total Warhammer III and a new miniatures game from Games Workshop, Warhammer: The Old World, which is also set for a late 2021 release.