From Faith Cometh Honor

IWs infantry advance, Obliterators on the spearhead.
Another specialization of the Iron Warriors used to be heavy infantry. Back in Great Crusade/Horus Heresy times, they used to have Tyrant Siege Terminator corps, who could make it through the breach under heavy fire while still blowing stuff up.
After Horus croaked and Traitor Legions had to flee to the Eye of Terror, they encountered a phenomena known as Obliterator Virus. This virus causes the afflicted to fuse with their armor and absorb any weapon they wield, allowing them to morph any of these on their arm at will (in Dawn Of War, they can fire either a Lascannon, Plasma Gun, or Assault Cannon depending on the target, in tabletop they also get Multimelta, twin meltaguns and a Plasma Cannon). First Obliterators emerged in Iron Warriors legion. While they are slow and sluggish in close combat, their raw firepower makes them a prized asset. Side effects of the affliction is pants on the head balls on the wall batsh_t insanity.