Ordo Chronos
The Ordo Chronus, referred to as the "Guardians of History" and possibly the Timehunters (or, thanks to their responsibilities, the "Clockblockers"), are a minor Order of the Inquisition. They specialise in studying the flow of time within the Warp, and studying and preventing time travel.
They are perhaps best known for vanishing en masse into the warp whilst trying to study it. Theories as to what happened to them vary: that they're chilling with the lost Primarchs and the Emperor in a hidden bunker, that they ironically ended up travelling far into the future and haven't turned up yet (or somehow erased themselves from the timeline), or that they became time units and went on adventures with their powers such as "Banish the Heresy", "Adjust the Light Clock" and "Observe the Sand Timer".
From the Architect of Fate anthology, a cell of inquisitors whom we could only guess are part of the Ordo Chronos have a throbbing hatredboner for Tzeentch, and especially Fateweaver, since those two like to fuck with causality so much.
They were originally introduced in Dark Heresy and Black Crusade sourcebooks, and were later canonised in Codex: Inquisition.
Andy Hoare, the order's creator, on his creation of them and ideas behind them.
