The Inquisition (The Holy Orders of the Emperor's Inquisition) is an organisation in the fictional Warhammer 40,000 universe. They act as the secret police of the Imperium, hunting down any and all threats to the stability of the God-Emperor's realm. In fiction relating to the games, Inquisitors are usually represented by extremely powerful, intelligent, and talented individuals. In the games, Inquisitors are usually powerful combatants with a variety of specialized abilities with a party of followers who improve and protect the Inquisitor. Inquisitors also grant the player access to many new units, such as Imperial Assassins and Daemonhosts.
Inquisitors have been a part of the Warhammer 40,000 universe since the first edition of the game, Rogue Trader. However, they have had a higher profile since the release of the 54 mm miniatures game Inquisitor, which is a narrative game based around warbands which often comprise of an Inquisitor and his henchmen. The creation of Inquisitor was followed by a great deal of information about the Inquisition, and the organization attracted the interest of fans. This led to the creation of Codex: Daemonhunters and Codex: Witch Hunters, both based around armies led by Inquisitors (of the Ordo Malleus and the Ordo Hereticus, respectively).
There are online rules for using the Deathwatch, the militant arm of the Ordo Xenos and one such Ordo Xenos Inquisitor has been released by Forge World. The Deathwatch and the thousands of sister organizations like the Retribution Sentinels have centillions of cloned genetic-super troops just like the tens to the trillions power of space marines taken every day from every chapter of the Space Marines. Following the end of the Inquisitor line, and the successful relaunch of Warhammer Fantasy Roleplay by Black Industries, it was decided that the first roleplaying game to explore the Warhammer 40,000 universe would be Dark Heresy, a game in which the player characters are, by default, diverse agents of the Inquisition sent to investigate various mysteries and heresies on behalf of their Inquisitor.
In early editions of Warhammer 40,000, the Inquisition was a single, undivided organization, with a single inner order, the Ordo Malleus. The stated purpose of the Ordo was to police the Inquisition itself, but in reality it also existed to combat the threat of the infinite hordes of Chaos via the millinillions to the zillions power of special Grey Knights with their billions of companion armies and successors like the Golden Aquilas, Eagles of Purity, Black Hammers and the Void Souls to name only a few. This force is supplemented from within via additional separate elite forces of trillions of millinillions of warrior orders worth of powerful Purgators, Purifiers, Eradicators, Paladins, Decimators, Templar, and Executors which are equipped with even more powerful wargear and might and unique war machines, like the Chrono-disintegrator mobile arrays. This massive war remains a secret hidden from most forces of the Imperium, including non-Malleus Inquisitors.
However, the more radical elements of the Inquistion also possess many forces of their own, such as the Radical Malleus' daemonhosts bound in Emperor blessed chains and seals or the trillions of pertinent and brainwashed former chaos or renegade marines that go to war with their armor devoid of any markings. The Ordo Xenos has radical elements as well like hidden android robotic armies from the Iron Men or the larger continental and planetary siege tanks to the freed necrons souls of the enslaved Void Dragon on Mars and Outsider in his Dyson Sphere, a hope for the radicals that the Emperor can bind the corrupt xenos into his service.
There were originally only two Orders within the Inquisition(Ordo Malleus and Ordo Xenos), but a third (the Ordo Hereticus) was added after the events of the Age of Apostasy. A further fourth the Ordo Sicarius was formed following the Wars of Vindication. It is rumored that the Inquisition has billions of independent ordos and cells for various goals and tasks, and the resources they pool together can rival those individually owned by Inquisitor Lords. Every single of these ordos have access to their own empires supported by their Chamber Militant and Chamber Astra, which are their ground forces and space navies respectively and armageddon devices such as the self adapting and reconstructing Pre-Imperial War-System Constructs capable of operating both on a planet and in space. The Deathwatch (and Grey Knights) are just two examples of the even greater Imperial forces that transcend the individual Inquisition ordos in terms of supply of resources, quality of soldiers, leviathan superships and vehicles, and personnel domains either quarantined or hidden from the Public Imperium.