'us'

Possible Cannon Theories
'us' it most likly refering to the spirts them self, we can infer this from lore revals from the sign. From the line where it says "A fraction of 'us' are now utillizing their newfound powers for many reasons; for plotting revenge, to find answers, or simply to entertain themselfs." we can infer that not all of 'us' are bad. The orgin of the spirts are proably the old people of the worlds that never respawned. From the Signs we know that the Random Obbies made them never respawn, but the location of where they came from is unkown.

Uncannon Theories
Though I theorize that the obbies came from ROoABH. This is not proven. I also think that it might have been the first obby, and it might have been what made the worlds in the first place.