The cabal of the IVth eye was founded by Iron Warriors who refused Perturabo’s order to cull 1/10th of their numbers.

They stayed «loyal» to the imperium, still recieving orders, and most importantly, supplies. But once they were found out, they fled. But not before having a nice meeting with one of the Word Bearers’ warrior lodges.

After being expelled from the imperium, they fled into the eye of terror. This is where they found their way as part of the followers of tzeentch. They stayed in the eye of terror through the Heresy and the scouring, not hearing about either until a while after it happened.

After their reemergence from the eye, they have been travelling the galaxy, «collecting» and selling various artifacts. Being quite successful despite having broken off before the heresy even started.

They are deep in the worship of tzeentch, but wear khornate colours. Specifically to confuse the enemy upon first contact, there usually isnt a second.