Forces of Chaos, bow to me.

In ages past, the Planet of Gerezman was the site of incursions from the forces of Chaos. The twilight days of the Horus Heresy saw civil war come to the planet. A warpstorm covered the world for many years, inviting daemons into real space to wreak havoc. Seeking relics dredged into the materium by the storm, the Warriors of Mayhem, a warband of Heretic Astartes, assaulted the world. Despite being driven off, they left behind carved altars and monuments to the dark gods, some of which still stand. The world is irreparably marked with the taint.

In the 4th edition Chaos Space marine codex, there were a number of wonderful colour schemes, brilliant for inspiration and given only very brief blurbs. I always found these more interesting than the standard, big name warbands. The Sanctified, the Death Shadows, and The Flawless Host are all found here. With at most a single sentence to describe them, these warbands took root deeply in my head. My favourite are the Warriors of Mayhem

They are offered without any description at all, but I have loved the colour scheme for years, using it for my own marine force and make my own segment of canon for them. I've put together some cult marines together, some test models for a new detachment.