The Watchers are group of unseen forces related to the down side up, as well as potentially the force that dragged the group into the older versions.
The watchers enjoy leaving clues for the Evolutionists in order to test there skills and problem solving.
They have also proven to be very vengeful beings punishing those who displease them or whom are not in their favor. This was proven when they put obsidian on top of all the chests in Grian's base.
They as well demand obedience and that there will be carried out. Though normally, they will leave the players to their own devices in order to observe.