A Deathguard Dreadnought
A Chaos Dreadnought is a Dreadnought in the service of the Chaos Space Marines. Under the influence of Chaos, many Chaos Dreadnoughts have degenerated into insane monsters, known as Helbrutes. However exemplars of ancient Dreadnought patterns still exist, having become dark reflections of their imperial counterparts.
The war saw the emergence of unique Dreadnought variants among the Traitor Legions that reflected this growing change. These included the Mhara Gal Tainted Dreadnought of the Word Bearers and the "berserker" Dreadnought of the World Eaters. The Mhara Ghal was the first to be utterly tainted by Chaos, its occupant a possessed of the Gal Vorbakwho had fused with the chassis.
After the Heresy, knowledge behind the creation of the old Dreadnought patterns declined among the traitors. With the exception of the prized Contemptor chassis, little effort was made to maintain and replicate the Legion's more sophisticated wargear. Without the technology to protect them from the ravages of time, such as stasis tombs, and exposed to the malignance that had spread among the legionnaires, internment within a Dreadnought's sarcophagus became a living hell. Under the sway of Chaos, many fell prey to insanity and mutation. This fate is also shared by the Dreadnoughts of Renegade Chapters that have succumbed to Chaos in the years after the Heresy, such as the Crimson Slaughter.
For Chaos Space Marines, incarceration within a Dreadnought is seen as a fate worse than death. Unsurprisingly, it is used as a means of punishment by their Lords.