Reading the Horus Heresy series of books, the diminuitive Watchers in the Dark make selective appearances in the books concerning the Dark Angels.
Background-wise, they appear at critical junctures: e.g. just prior to the Emporor arriving, and so are believed to be heralds of great things. They appear to be impervious to physical assault and warp related events, they are probably powerful psykers; and more-over may even have lectured the Lion about chaos. So my question for crowd-sourcing today is this: are they some kind of strange daemon? I've heard it said that they might even be Cabal agents. Or maybe they are something else altogether that is native to Caliban? Thoughts anyone?