An Inquisitor is a clandestine agent and highest-ranking member of the Imperial Inqisition. They are the secret and intenlligence agency of the Imperium, driven by ancient order of the Emperor of Mankind to ensure the security of the Imperium of Man from the taints of Chaos, the daemonic threats of the Warp and the dangers presented to Mankind by intelligent alien species like the Orks, the Tyrainds, The Eldar, the Necrons and the Tau. Split into three main Orders, the Ordo Xenos (Alien Hunters), the Ordo Malleus (Daemon Hunters) and the Ordo Hereticus (Witch Hunters), the Inquisition is ruthless in uncovering and snuffing out Chaotic corruption, heresy, and mutation whereever they may be found among the million worlds of Mankind. The relentless and deadly agents of the Inquisition have infiltrated every facet of the human society to insure that Chaos can never again take root in the Imperium and that no alien race will replace humanity as the dominant intelligent species of the Milky Way Galaxy.
An Inquisitor of the Ordo Malleus in Power Armour
Inquisitor Gregor Eisenhorn of the Ordo Xenos
An Ordo Hereticus Inquisitor, armed with Power Sword and Inferno Pistol
As the most important arm of the Imperial government dedicated to ensuring the security of the Imperium of Man, the authority of the Inquision technially supersedes all others, including, in theory, a High Lord of Terra or even a Chapter Master of the Adeptus Astartes. Thus, Inqisitors have access to an almost limitless variety of Imperial resources that allow them to act in full strength against any threats they perceive to the Emperor's domain. Despite their seemingly powerful capabilities, even the Inquisitor may not be able to fight back a rampant and well-rooted infection of Chaos or unusually powerful xenos forces like a Tyraind Hive Fleet. On worlds where such infections have been discovered, an Inquisitor may condemn a planet to Exterminatus, the ultimate Imperial sanction which is granted only to an Inquisitor or a Space Marine officer to unleash upon a world. Exterminatus means the world will become a Dead World which is done in many ways.