From Warhammer 40k - Lexicanum
The Secret Police of the Imperium, the Inquisition was created by the Emperor of Mankind in the twilight hours prior to His internment within the Golden Throne. As Terra lay besieged by the traitor forces of Warmaster Horus, the Emperor commanded Malcador the Sigillite to seek out "men of character, skill, and determination" to be tested and trained so that an elite group of investigators might be tasked to discover the alien, mutant, and heretic.
The Inquisition is broken down into three major Ordos, or Orders, and they are:
They are aided by the lesser Ordos (such as the Ordo Sicarius, which regulates the Officio Assassinorum). Each of the three Ordos has an "Order Militant", a fighting force trained to fighting the chosen enemy of their Order:
Inquisitors and their servants fall into a series of ranks:
A deeply philosophical organization, the Inquisition is also divided along a series of ideologies and philosophies that fall under two broad types:
The Emperor's Wrath Incarnate
The vehicles and troops available to the Inquisition come from every agency within the Imperium, including Space Marine Chapters and Imperial Guard Regiments, because Inquisitors are the supreme force of the Imperium and are able to requisition the aid of anyone on demand, without question.
Codices and Background Books
The Inquisitor game is devoted to the characters, background, and intrigue of the Holy Orders of the Emperor's Inquisition and its battle against the alien, mutant, and heretic. The player is tasked with rooting out plots against the Imperium before it is too late.