Watchers in the Dark
The Watchers in the Dark are
Jawas diminutive xenos who follow around the Dark Angels. No one knows what they look like, for they are covered in large green robes and resist all psykers that try to scry them. Also all of them look a bit like Orko from the Masters of the Universe (AKA He-Man, one of the most homoerotic shows for kids. Heck they even have a character named Fisto, ooohhh He-Man, stay classy). While Chaos has human ones.
The Watchers are extra-dimensional creatures and only take their diminutive appearance out of habit when they enter the corporeal reality. The dimension where they originate from is not named or described, but the Watchers do not appear to be warp-based creatures. Wherever they come from, metrics such as "time" and "space" seem to be entirely irrelevant to them and they were able to view time as a three-dimensional mosaic, yet were incapable of comprehending it in its entirety; however, they could hone in on particular details and determine their significance and try to influence events in subtle ways. Additionally, they were able to appear and disappear wherever and whenever they like, making them impossible to capture or sneak up on. In short, while they are not from the parallel universe known as the Warp, they do seem to originate from a higher dimension, essentially making them an entire race of G-men.
History and Shenanigans
The Watchers were first known to the people of ancient Caliban as mysterious boogeymen who wandered the forests. In reality, they were a xenos race implied to be members of, or associated with the Cabal, a group dedicated to fighting Chaos. They had been sent to Caliban to guard the Ouroboros, acting as its "jailer" and apparently prevent its abuses. By pure happenstance, humanity arrived on the planet during the Dark Age of Technology, and the Watchers had been "watching" humanity on Caliban for over 15,000 years, which means their influence probably predates the Fall of the Eldar. Regardless on the exact timing of the human colonisation against formation of the Eye of Terror the planet became contaminated by Warp energy due to its proximity, resulting in the Great Beasts who prowled the forests and a terrible time for all involved.
The Watchers certainly knew of the activities of the Cabal, specifically that Eldrad had attempted to pre-warn Fulgrim of the dangers of Chaos which went unheeded, though the Watchers had also attempted to give their own warnings to the Lion and various members of the First Legion, especially when the rest of the cabal specifically tried to get Alpharius and the last legion on board with them.
The Watchers were also apparently aware of the potential outcome of what would happen if the Emperor's forces won during the Siege of Terra (ie: the galaxy stagnating and Chaos growing in strength) though the Watchers also had a difference of opinion with the members of the Cabal in that they believed "Chaos could not be defeated; only fought", which puts them at odds with the notion that Chaos could somehow be defeated once-and-for-all if Horus actually won. The Watchers seemed more interested in what happened to the Ouroboros than attempting to influence the outcome of the Horus Heresy to one side or another. In any case, the Cabal was later eliminated by Eldrad when he figured its members were doing more harm than good after he realised that Perpetuals were interfering with their ability to accurately predict the future; Eldrad left the Watchers alone, implying that he either could not do anything to them, or they were not full members of the Cabal.
However, they saw it as their duty to try and warn those in a position to do something. Although they were known to have communicated with the Lion in the past, his constant reliance on Tuchulcha is preventing them from contacting him directly, and so they spoke to various proxies such as Zahariel and Farith Redloss in the hopes that they could get the Lion to change his mind.
Through a series of still unconfirmed events (which will likely be revealed in future Horus Heresy books; GW is nothing if not practiced at draining your wallet) the Watchers in the Dark seem to take a considerable demotion from entities unbound by linear time and location, dropping to "serve" the Dark Angels as retainers and weapon bearers. They also have a curious interaction with Cypher and were noted to change their behaviour when he visited The Rock, they are presumably also the reason he constantly escapes each and every time he gets captured. Finally, they are also said to look over the comatose body of Lion El'Jonson, asleep in the Rock.
As members of a faction dedicated to the war against Chaos, regardless of their effectiveness in doing so, they are a big piece of evidence for the Dark Angels not being a secret Chaos Space Marine Legion in disguise. During Wrath of Magnus when the Changeling fled through the Rock once it was discovered, it ran headlong into a Watcher carrying a Crozius on its form. The Changeling was horrified upon meeting it, recognising it as some sort of anathema to its kind. Were they ever to turn into Chaos Marines, logically the Watchers would leave, presumably taking the slumbering Primarch with them.
Caliban: The Predestination Paradox
"Perhaps, but I fear that the destruction of Caliban is our last hope for the future now. It will be the final blow that will sunder them. Is even the Lion willing to commit such an act?"
- – one of the Watchers in the Dark, viewing the end of the Siege of Terra
When the Primordial Annihilator was defeated on Terra, a group of Watchers lamented that Chaos would run rampant and merely arise again on Caliban. After examining the potential futures in the mosaic of time, their collective plan to save the galaxy was to engineer the destruction of the planet, creating the only outcome they believed would work, although how this plan affects the future in any positive way has yet to be explained.
However, the plan to destroy the planet would not actually come to fruition until ten thousand years later when Cypher visited the Rock, got Azrael to agree to travel to the ruins of Caliban where the three elements: Ouroboros, Tuchulcha and the "Plagueheart" held by Typhus would meet and create a portal back to the time of Luther's insurrection.
Azrael would then destroy the time portal before the Death Guard made their way back and/or the Fallen Angels made their way forward through the time stream to influence events and royally screw up the "normal" sequence of events, though destroying the portal seemed to have the inadvertent affect of also destroying the planet in the past.
This creates a predestination paradox: as the period from the 31st Millennium though to the 41st Millennium would have likely first existed in a time-stream where Caliban was not destroyed up until a traveler from the future could initiate events to destroy the planet of the past, making one wonder what that initial outcome would have been like for the galaxy... The Watchers response indicates the survival of Caliban is probably the worst case scenario, while Ezekiel reassures Azrael that it is probably for the best not to think too hard about it.
But it does demonstrate the potential for changing history within the setting itself.
They appear/do not appear on the table, as literal place holders for the Dark Angels; Azrael comes with one which carries the Lion Helm, with Company Master and Deathwing Knights having the option to take them. Though in the last two cases they grant Fear and Adamantium Will. We can say they do not appear as they are purely for show and don't do anything but symbolize wargear, with the option to not use their model at all or remove them if they get in the way.
As of 8th Edition, the Watchers are still here but with the changes in rules their function changed as well. Azrael's minion still carries around the helmet like before, but now the Watchers that accompany Deathwing squads provide a one time chance to nullify an incoming enemy psychic power on a 3+, which is more reliable than using a psyker to deny powers.