Psi bombers were rogue psychers tormented and augmented by the Stygian Empire and their Alpha Legion overlords. These psychers were given drugs to augment their abilities often at the cost of their sanity and long term health. They were taught words of power and summoning and then smuggled behind Imperial lines and unleashed to create havoc.
During the Battles for Guttar, they were often smuggled via sewer lines behind the imperial frontline. Then they would unleash their power bridging the gap between reality and the warp and allowing the passage of bands of daemons to rampage behind the Imperial line. Often the unleashing of such power would kill the rogue psyche in the process. However the damage they were able to inflict at the cost of their own lives was often horrendous.