R'lyeh

R'lyeh is a sunken city in the Pacific which is featured in H.P. Lovecraft's short story "The Call of Cthulhu". Imprisoned within it is the entity known as Cthulhu.

Principles
The underwater city of R'lyeh, in which Cthulhu is imprisoned, is said to be sunk at a certain point in the Pacific. The city architecture is said to be of non-Euclidean geometry.

According to the story, when certain conditions are met, the city would rise to the surface from the bottom of the ocean. When R'lyeh surfaced, it was said that the weak-minded would go insane while poets and artists would receive strange inspiration. These conditions include the positions of the stars and submarine volcanic activity.

As with other spells based on the Cthulhu Mythos, spells involving the city are classed as creating something from nothing - calling into existence something which shouldn't exist.