Originally Posted by Belthazor Aurellius
Here's my take on what could work for you.
Pulled from almost every traitor legion, the Black Knights are one of Fabius Bile's successful modifications. The legionaires chosen had to pass new trials, and were pitted against rage, plague, sorcery and pain. The few who survived the indoctrination to Chaos Undivided, were genomodified to amplify latent psychic powers. Their black carapaces were etched with blasphemous runes of power. Thier armor was forged anew into a mockery of the Grey Knights of the Imperium, with unholy icons gleaming brightly along the edges of their shining sable armor.
These Black Knights appear unexpectedly at the height of Imperial Mass, when a living saint is raising her voice to the Emperor, or when a priest is raising his voice against heresy. Working in conjunction with traitor cults in the midst of these congregations, their timing is precise, their arrival unexpected and the endgame horrific for any martyr to the Emperor.
These highly trained warriors of the Undivided pantheon strike down those thought invulnerable with their modified Kai Guns, which cause rosarii to overload and iron halos to crack.
Yet, these Black Knights abhor their erstwhile allies. Those who are not as pure in their devotion to Chaos, the traitor marine legions. As such, they abstain from allying with them except in the most dire circumstance.
Black Knights may take disciplined traitor guard cultists as allies (inducted imperial guard), and may from time to time, be led by a Dark Master (Inquisitor)... These are feared men, as they often work for Abbaddon or his generals to weed out the impure followers of Chaos.
Could see this working fluff-wise! Have some +rep my friend!