The Hellsmith is the Chaos Space Marine-Version of the Techmarine. These Smiths are (not the guys from the Matrix) the Lords of the Chaos-Forge and have mastered the Art of Warp-forging for the last ten thousand years. Amongst their creations are some of the most infamous warp-crafted Powerswords, Halberds, Warp-tainted Bolters and Demonmauls ever to be wielded by the Chaos Lords of the Traitor Legions. Apart from forging Melee-Weapons, they're also known for creating some of the most terrifying Warp-Engines in service of the Ruinous Powers. They also have a reputation for repairing any kind of vehicle, building, defence or armor far faster than most Dark Mechanicum Adepts.
In contrast to their weaker predecessors, the Heretics, the Hellsmiths are capable of defending themselfs with a variation of weapons. Like their loyal counterpart, they can construct buildings much faster than the standard builing unit, they're however command type units and only one per army will be available.