The Knights of the Watch are not a standard chapter but a fellowship of veteran Astartes who, inspired by the example of Nathaniel Garro, have left their Chapters to follow the path of Errantry, seeking out and destroying the enemies of the Emperor wherever they might be found.
The origins of the fellowship of the Watch are uncertain but Astartes wearing the white and disclaiming connection with any Chapter have been documented as far back as the immediate post-Heresy period. Some theorize that the original Knights were loyalists from the Traitor Legions who shed all former allegiances and dedicated themselves to the Emperor alone through his servants the Lords of Terra.
Organization and Mission
The Watch is stunningly un-Codex compliant - possibly explaining the Ultramarines' general hostility to the fellowship. Such organization as they have is based on a system of small watch stations, nineteen of them, sited at strategic points throughout the galaxy usually at the nexus of several warp-currents for ease of communication. Each station is headed by a Knight Commander supported - when possible - by a trio of specialist marines; Librarian, Techmarine and Apothecary. The Commander oversees a small staff of Watch-servants (their version of Chapter Serfs) comprising an Astropath choir and an eclectic collection of experts and specialists whose job it is to analyze the constant stream of intelligence incoming from Knights and Friends of the Watch. The station provides hospitality for any Knights operating in its zone and the Commander will pass on intelligence to same and even at times assign missions.
Most Knights belong to companion bands usually numbering between five and ten marines. Band organization is informal in the extreme and leadership may shift from knight to knight depending on the situation and skills needed. Most bands include a number of Watch-servant auxillaries. The elite Knights Errantus operate solo except perhaps for a Watch-servant or two. The Watch is a roving force ranging at will through sectors and station-zones pursuing the Mission of the Watch. That mission as set down millenia ago, possibly by the Emperor Himself, is to seek out and eliminate any and all threats to the Unity of Mankind, ideally resolving small problems before they become big ones. Needless to say they do not always succeed.
The Watch has no equivalent to a Chapter Master. Instead such central control as there is is exercised by a council of ten Captains General, Knights Commander or Errantus selected by their Brothers two from each Segmentum, who meet periodically at Watch Station Prime on Proxima Void Station orbiting Proxima Centauri.
The Watch's method of recruitment involves a long period of observation of a candidate, who is usually quite unaware of their interest, before they invite him to take the Watch-Oath. Chapters are not required to yield their Brothers to the Knights as they are to the Deathwatch and how amenable Chapter Masters are to releasing a Brother varies, the Ultramarines for example disapprove of the Watch and rarely permit brothers to join. The Blood Raven chapter on the other hand is very ready to extend such permissions. It helps that the kind of Brothers who interest the Watch tend to be non-conformists and even outright troublemakers that their Masters are quite willing to see the back of. The number of years to be served is often set in the oath itself but sometimes the recruit - with the permission of his Chapter Master - leaves it unstated. Some Knights eventually chose to serve life long. Frequently a Marine will appear at a watch station volunteering himself. If he passes Knight ordeals he is accepted. Many of these men join for life without revealing their former allegiance.
A high proportion of Librarians and Tech-marines - who often feel somewhat alienated from their Chapter brothers - take the oath. Every band of Knight Companions will have at least one or two Tech-Marines and many Librarians become Errantus.
The newly chosen Knight paints his armor bone white except for the right pauldron which is left in its original colors. The left pauldron, vambrace and gauntlet are gold plated and the pauldron embossed with the icon of the Watch: A gold gothic cross on a red circle with a laurel wreathed skull in the center. If he takes the life-oath the right pauldron is also painted bone color.
Operations of the Watch
Operating as they do singly or in small groups very thinly spread throughout the Empire it is perhaps unsurprising that the vast majority of the Emperor's subjects are unaware of their existence. The Watch's goal is to eliminate potential threats while they are still small so their exploits are rarely noticed beyond the immediate circle of those affected. As the Knights prefer to operate under the radar they have absolutely no problem with this. In fact they tend to regard public notice as a sign of failure. It means they let the problem get too big.