The Undivided is a name for the Chaos hosts that serve under no god. In Mark of Chaos's Chaos campaign, you will get to choose between fighting under the God of Warfare, Khorne, or the God of Filth, Nurgle. the undivided are going to be your most commom units at the start of the game. In the second mission you come across the warriors of khorne. There is not much differents between the Undivided and the worriors of Khorne and Nurgle.
Within the Xbox 360 version, Chaos Undivided are the chaos host that serves under no god, or can be considered to serve all of the Ruionous powers equally. Differing from the Chaos Factions of Khorne and Nurgle, Chaos Undivded specializes in the dark magic, utilizing the powers of their Sorcers and other creatures. Unlike the other two gods, Chaos Undivded does not have their own specilized demons, only being allowed to used the mutated creates such as Wyrmspans or Spawns of Chaos.