Southern Realms refers to a group of minor human factions who are grouped together in diplomacy. In Total War: Warhammer this consisted of Tilea, Estalia, and the Border Princes, and was sometimes referred to as TEB.
These factions have their own separate cultures in Warhammer Fantasy lore, but in the games they currently use units from the Empire unit roster, and if Telia, Estalia or Border Princes generally only have access to tier 1-3 level fo troops (Sudenburg and New World Colonies have access to all units). They also use the Empire buildings, however instead of the Temple of Sigmar they have a Temple of Myrmidia.
Due to being classified as a seperate kind of faction they cannot be confederated with by any Empire playable factions.
Southern Realm Factions
Introduced in The Old World campaign:
Introduced in the Eye of the Vortex campaign:
Introduced in the Aye-Aye! Patch: