Tarnhelm was a stealth ship used by the Knights-Errant during the Horus Heresy. Built on the orders of Malcador, the Tarnhelm was a small Warp Drive-equipped vessel, too large to be an Attack Craft but too small for a patrol ship. The Tarnehlm was purpose-built on Titan to infiltrate the Sons of Horus flagship Vengeful Spirit and land a team of Knights-Errant so that the traitor vessel could be marked for a future assault by Leman Russ. The operation was launched after the Battle of Molech, and the Tarnhelm successfully got within range of the Vengeful Spirit to board Malcador's Knights. However the team was discovered and engaged, and the Tarnhelm was forced to fire into Lupercal's Court, creating a distraction that allowed the Knights-Errant to escape.