In detailing what qualifies as a Total War Saga game, CA describes it in their FAQ from earlier this year as being bigger than a standard Campaign Pack while not being an entirely new era-spanning Total War game like the main entries in their franchise.
Taking place in 878 AD, the game pits the rulers of Britain against each other in battles to control the country, with this spinoff title being more directly focused on those exact moments and times as opposed to the generation-spanning conflicts other Total War games have involved. While all the factions are yet to be revealed so far CA confirm that Viking settlers, Anglo-Saxons and "certain Gaelic clans" will be among them. Britannia is a furnace of possibilities, from which the template for modern Britain will be forged, across the most detailed Total War campaign map ever made.
Creative Assembly is one of the UK's leading games development studios, founded in 1987 and located in West Sussex, UK and in Sofia, Bulgaria. It shouldn't be surprising that world history has a lot of sources to pull from.
Today, SEGA announced the very first game in this new series, Thrones of Britannia. Alongside the core historical-based games, the Total War series has expanded to include free-to-play mobile (Total War Battles™: KINGDOM) and PC titles (Total War™: ARENA), now in development. There's no price or date for the game as of yet, aside from the 2018 window.