The Watchers were motionless Sith statues found around the planet of Dromund Kaas and inside the Dark Force Temple. Their appearance was like stone slabs, crudely carved to resemble a cloaked Human figure, holding a sword (perhaps a Sith sword).
Their purpose was to guard the swamp and places on the way to the Temple, blocking or harming unwanted visitors. In some cases, their eyes glowed.
Although motionless, the statues appear to eternally watch their ground, and react to someone's presence. They seemed somehow sentient or at least enchanted with some obscure power. The only way to pass through their traps was the use of Force Persuasion.
Some of them were there for purely cosmetic reasons, or to indicate an important place or entrance. But even then, the impression that they were continually standing watch was there.