If you've delved into the comics included with the collector's edition of the original game, it provides a clearer context. The leader of the Raven Host, Tchar-whatever, unites the fractured tribes of Norsca, all of whom venerate different chaos deities. His rise to power means we'll encounter NPCs representing all the Chaos gods, including followers of Khorne in places like Bastion Stair.
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Essentially, the Raven Host can be seen as a chaotic, undivided faction, with Tzeentch as the mastermind behind their schemes.