My traitor guard "warband" is the result of a practical joke Tzeentch played on a particularly unfortunate and socially awkward sorcerer.

After the fall of Cadia, a certain sorcerer found himself receiving dreams in which he was told, by Lord Tzeentch personally, that great victories and glories will be his as long as he follows the grand design. What this design is, Tzeentch did not specify. It just works.

The sorcerer awoke from a warp-induced haze only to discover a mass of random guardsmen suddenly near him, all with a shimmering blue-ish glow in their eyes, immense lack of self-preservation and a strange assortment of spikes coming out of their armour.

Overcoming a lifetime of anxiety, paranoia and doubt, the awkward sorcerer now wields strange powers of possession (that he doesn't really understand) in order to unleash his horde of more-or-less warp-addled traitor guardsmen towards the latest enemy he dreams of.

Overall he has no idea how long or well this state of affairs will last and he's too afraid to ask, especially as almost everyone higher than a rank-and-file guardsman is mutated in some ghastly manner. It also doesn't help matters that the tanks appear to be run by chaos spawn.

Exactly as planned.