Yog-Sothoth

Yog-Sothoth is frequently said to be conterminous with all space and time. While this is undoubtedly true on some level, this terrifying entity cannot physically intervene in the waking world unless summoned. When a magical gate is opened for his passage, Yog-Sothoth can intrude into the mortal realms temporarily, to seek sacrifice or to advance his cause, but usually only as a fragment of his true self. For his whole reality to enter the waking world, a gate of colossal size must be prepared, that Yog-Sothoth might enter fully and bring about the end of the world. Fortunately for life and sanity, this is difficult to accomplish and requires significant work.
Yog-Sothoth is an utterly alien being, sometimes described as a congeries of spheres, but the reality is far more terrifying. These "spheres" are bladders or spawning sacs, and the whole is loosely knitted together with rootlets to form a semi-coherent body. The individual sacs can change their three-dimensional relationship with one another, and even operate normally when separated at a distance, but typically hang together. Sometimes Yog-Sothoth can separate a chunk of himself to act semi-independently and even travel in the mortal realm, but this typically lasts only for the length of time his gate passageway is opened (rarely more than a few hours).
Yog-Sothoth is able to fly, travel between dimensions, and grant such travel to his minions and those who seek him out.
When Yog-Sothoth is called forth to take on physical form, he must literally squeeze through the portal that is opened for his presence. Except in extremely rare cases, the gate permits only the protrusion of a small part of Yog-Sothoth: huge compared to a humanoid and potent magical and physical threat. Yog-Sothoth is often summoned forth to teach dark secrets to his minions or worse, to intermix genetically with humankind, giving rise to outer mutants, abominations, or spawn. When he does so, he usually requires sacrifices.
When seen in his aspect as the unreality between dimensions, Yog-Sothoth is known as Umr at-Tawil and is covered by what appears to be a cloth. This is probably his least malign manifestation.

Outer God of arcane gates, time, and space; Chaotic-Neutral

Favored Weapon: Dagger
Symbol: Black Spiral
Temple: Hilltops, observatories, orreries, standing stones, stone towers
Worshipers: Astronomers and stargazers, deranged spellcasters, fringe scientists, philosophers
Servitors: Outer mutants, outer abominations, outer spawn