Watchers in the Dark
The Watchers in the Dark are 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.
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 the Cabal dedicated to fighting Chaos (but really are little more than super-dicks who watch the galaxy burn, and even helped make it happen). They had been sent to watch over Caliban, which had been contaminated by Warp energy due to its proximity to the Eye of Terror, resulting in the Great Beasts who prowled the forests. 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 slowly) the Watchers in the Dark serve the Dark Angels as retainers and weapon bearers. They are also said to look over the comatose body of Lion El'Jonson, asleep in the Rock.
They appear/do not appear on the table, as literal place holders for the Dark Angels. Azreal comes with one which carries the Lion Helm, with Company Master and Deathwing Knights having the option to take them. In the last two cases they grant Fear and Adamantian Will. We say do not appear as they are purely for show and dont do anything but symbolize wargear, with the option to not use their model at all or remove them if they get in the way.