"It was everywhere at once, yet nowhere... how could we fight against such a thing? Visions of countless realities flashed all around us as the great monster drew us into it's clutches and attacked; I could only watch with despair as several of my comerades were torn to shreds as the very air seemed to rip apart like paper. Despite our strongest assaults, hope was beyond our reach. There were no miracles, in the end. Only Yog-Sothoth."
Yog-Sothoth is one of the Outer Gods that reigns over The Dreamlands. It takes the form of countless conglomerated globes, ensnared and held together with tentacles extruding from great cavernous maws. It co-reigns over the Dreamlands with Azathoth, though it is a more neutral being.
Yog-Sothoth guards the entrance into the Dreamlands, being able to open and close "the gate between worlds", but requiring a "key" to do so. This key is simply described as a "source of imbalance", and it is what Morpheus seeks, for once the gate between worlds is opened, the Outer Gods will awaken and become free.