The Ordo Xenos was set up by the Emperor shortly before the Great Crusade, to investigate alien races, and destroy any that are deemed to be a threat. Later the Ordo Xenos' jurisdiction extended to cover Navigators. The Deathwatch is their Ordo Militant. The Deathwatch are unique amongst the Space Marine Chapters in that their recruits are seconded to them from other Space Marine chapters. Whether this will change when the Ordo Xenos gains it's own Codex is unknown.