In the mirror universe, an Assassin of Q was a murderous agent of the Orion Cabal, a cult of Orions devoted to their evil god Q. The Assassins carried out the Cabal's commands to spread fear and suffering, though they could also be hired by non-Cabal members for assassinations, usually through the Procurer's Guild.
According to legend, Assassins of Q were high-caste Orions who were granted special powers by Q at one of the Cabal's sacrifices. These powers were supernatural, incomprehensible to science, allowing Assassins to do such things as turn invisible, pass through walls, dodge phaser beams and survive in the vacuum of space. Whatever the truth, they were known as terrifying killers that rivaled the Ferengi zakrim.
The popular image of Assassins was of robed and chanting Orion zealots, smelling of incense and brimstone. However, real assassins were less obvious, and usually disguised as common people, for example, as a beaten slave, a bored worker, or a sickly beggar. (Decipher RPG module: Through a Glass, Darkly)