The Inquisition, formally called the Holy Orders of the Emperor’s Inquisition, is a secret organisation that exists outside the standard administrative hierarchy of the Imperium of Man.
The Inquisition acts as the secret police force of the Imperium, hunting down any and all of the myriad threats to the stability of the God-Emperor’s realm, from the corruption caused by the forces of Chaos, Heretics, mutants and rebels, to assaults from vicious alien species like the Tyranids, Orks or Drukhari.
The Inquisition is divided into three major orders or “Ordos Majoris” in High Gothic and many other minor ones, or “Ordos Minoris,” each dedicated to confronting a very specific threat to the continued survival of the Imperium and Humanity.
The Inquisitors of the Ordo Malleus and their Chamber Militant, the Space Marines of the Grey Knights, seek to confront and defeat the forces of Chaos and the other threats from beyond wherever they assault Imperial space.
The Ordo Xenos and its Chamber Militant, the Astartes of the Deathwatch, defend the Imperium from the myriad alien threats that lie outside its borders.
The Ordo Hereticus and its Chamber Militant, the pious battle-sisters of the Adepta Sororitas’ Orders Militant, seek to combat all of the very Human threats to the Imperium that lie within its own organisations or on its own worlds.