Many games have something that fits the definition of Enigmatic Watcher. In Pathfinder, quite a few outsiders can fit the bill, although the best are the Aeons that strive to keep the multiverse balanced. Numenera has the philethis. Eclipse Phase has the concept of sousveillance, where the entire world is wired with sensors and cameras and anyone can be watching at any time. It’s a useful and powerful trope, one that can evoke either a sense of presence and destiny or a sense of paranoia, depending on how it gets played.
Aeons and the Philethis are creatures whose presence, if understood, lends a weight to the events unfolding; here are creatures whose motives are unknown but who are whispered to have a hand in the unfolding of fate and chance. Watchers like these are best used sparingly – a single aeon hovering silently in an unremarkable corner, only noticed by those specifically looking, or a philethis looming on top of a nearby hill, watching the players until they get close, then fading away like smoke on a breeze. The only rolls players should make with this kind of Watcher is the roll to spot them in a scene, to realize that something momentous is underway.
The other kind of Watcher is the kind that induces paranoia; used best by letting players glimpse it from a distance, playing up the way it seems to be looking right at them, a few times in a row. Followed up by red herrings – a shadowy figure in a room turns out to be a tailor’s mannequin in a coat, or the shuffling sound in the dark is just a rat nosing around a discarded meal – it can ratchet up the wariness and paranoia of the group as they start keeping an eye out for anything unusual.
The key, of course, is to decide which kind of Watcher you want to include; campaigns can have both, but that requires a particular kind of GM and a great deal of skill to pull off. For most, you’ll need to think things through and decide what kind of feel you want your Watchers to evoke. Anticipation and concern about what lies ahead? You want the enigmatic Watcher who follows destiny and fate. Paranoia, uncertainty, and distrust? You want the Watcher in the shadows.
Using the first type is simple; you’ll simply add them to any game map you have, leaving them in an unobtrusive spot for the players to notice if they look around. Some GMs simply place them as a mock corner weight to hold the map in place, occasionally adjusting them as if they’re smoothing the map as they make sure the figure keeps facing the PCs. Don’t use the figure if the players aren’t doing something important. Start messing with them just a little whenever the figure is out; give them a very slight and unexplained bonus or penalty, as the Watcher subtly ‘guides destiny’ without seeming to be involved. Eventually, someone will start to notice a pattern to the figure’s appearances and their unexplained good or bad fortune.
The second type of Watcher is more difficult, but also easier to get the players to pay attention to; begin with a few small tells, such as an odd cloaked figure staring at them from some distance away, a particular scraping sound from somewhere dark behind them, or a flickering light in the distance at night. The important thing here is to have it be a small but reliable tell, infrequently enough to not make it an instant alarm, and to make it hard to go investigate. Escalate it gradually, so that when they turn at the sound, there’s a glimpse of a dark figure moving out of sight, or the odd figure turns up in places it shouldn’t be – outside a window with no balcony or ledge, on the dais next to the king (and no one else seems to notice it), behind the bars of a locked room, only to be gone when the players try to get to it.
In both cases, the Watcher needs to have an agenda; the ‘destiny’ type want the PCs to go to particular locations and do particular things in order to fulfill a cosmic role or make sure that things go in accordance with a Plan that covers all of existence. The creepy type have something much more local and specific in mind – they’re often minions of some greater power, tasked with keeping an eye on the characters, but they can be villains themselves – or, on occasion, unexpected allies whose alarming natures stem from a misunderstanding of human nature.
In both cases, the role they play relies on them remaining out of the reach of the characters. Once they’re forced into the open – and if the Watcher is being done right, forcing them into the open will become a priority for the players – they become something else – an open ally, an open foe, a strange source of information, or any other number of things. Be prepared for the players to try, and be prepared to foil them for a time. Let them get closer on each try, and don’t tease them for too long or the paranoia or confusion will turn into annoyance and irritation.
And that’s no fun for anyone.