The Heretic is the main builder of the Forces of Chaos. These Chaos Worshippers are wreckaged souls, tortured by the powers of the Warp and now serving their dark purposes. Heretics are being used by the forces of Chaos as the first invasion force next to Cultists and Renegades. While the Cultists worship the Ruinous powers of Chaos in order to gain their favor for the upcoming war and the Renegades form the first trained military strike-force, the Heretics "manipulate" the Warp to summon the constructions of the true Champions of Chaos: The Chaos Space Marines. Once the Chaos Marines are on the battlefield, the Cultist and Renegades become more or less useless and therefore are from that point on merely used as meat-shields for the Marines. As such the Heretics remain on the battlefield as the only mortals with a higher purpose than being a meat-shield.
Just like the Servitor of the Space Marines, the Heretic is incapable of defending himself against hostile forces. However, in comparison to the Servitor, the Heretic can be forced to work harder, like a slave. This however comes at the cost of HP. So if you activate the ability "slave worker" and forget to turn it of, the Heretic simply dies through overworking. And just like in the Space Marine Faction, the Heretic also has a "bigger brother" that is known as the Hellsmith.