The entity known as the “Key and the Gate” in Lovecraft's stories is Yog-Sothoth. The phrase reflects the entity's association with all time and space, along with its ability to exist beyond ordinary physical reality.
Yog-Sothoth first appeared in Lovecraft's 1926 novella The Case of Charles Dexter Ward, where the name is part of an occult invocation. The entity also plays a major role in “The Dunwich Horror,” in which it is connected to the Whateley family's supernatural plans and to the monstrous Wilbur Whateley.
Unlike Cthulhu, who is often portrayed as a dormant being beneath the sea, Yog-Sothoth is generally presented as a cosmic intelligence or force. Lovecraft's descriptions vary, but recurring imagery includes glowing spheres or globes that suggest a form beyond human comprehension.
A common mix-up is confusing Yog-Sothoth with Azathoth, the blind and mindless center of cosmic chaos, or Nyarlathotep, the active messenger and shape-shifter. “Key and Gate” is the distinctive association of Yog-Sothoth.