Daemonhunters

The greatest threat to the Imperium of Man is, and has always been, the followers of Chaos and the foul daemons of the warp. The Inquisitors of the Ordo Malleus are at war with forces too terrible to comprehend and, together with the holy warriors of the Grey Knights, they must take the light of the Emperor into the darkness. The threat of the daemonic is so great that only these heroic warriors have the necessary skills and knowledge to stand against such diabolical foes..

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The Ordo has a complete Chapter of the Legions Astartes attached to it on a permanent basis. The Grey Knights were a single Chapter created during an unregistered Founding shortly after the (official) Third Founding. Although technically Marines of the Adeptus Astartes, the Grey Knights are, to all intents and purposes, part of the Inquisition. They are listed as a Third Founding unit, and by the Emperor's instruction, were designated Chapter number 666.