The Tetrad

The Tetrad is common name for the universe: the sum totality of space, time, and matter. It is named so for the four planes that compose it, which are in turn made from the pairings of the four fundamental elements. These are Lampetia (Air and Fire), Nephele (Air and Water), Adrastea (Water and Earth), and Hekate (Earth and Fire).

Physical Properties
The Tetrad is spherical, with its planes stacked vertically on top of one another in this order, descending: Lampetia, Nephele, Adrastea, and Hekate. Naturally, this means that the deeper the plane, the smaller, with Lampetia equaling, or even dwarfing, every other plane combined in size, and Hekate being perhaps one-third of Adrastea's surface.

While the Tetrad by definition contains everything that exists (grouped together under the umbrella substance of "Light"), the nonexistent (entities beyond space, time, and matter known as "Darkness") can on occasion breach its fabric. As Light is self-replicating, such tears are usually shored up, but big enough gaps can persist and even worsen. The greatest breach occurs in Lampetia every 24 hours, in which Darkness enters it for 12 hours: this phenomenon is known as Night.

Lampetia
Lampetia, world of Air and Fire, is the highest and largest realm. It primarily consists of a void, inhabited only by cosmic dust, the occasional comet/asteroid, and enormous entities of Air and Fire known as the Celestials: the impossibly massive Sun, the transforming Moon, and countless Stars. The most powerful beings in existence, they use every ounce of their fathomless might to battle the forces of Darkness constantly.

The largest breach in the Tetrad occurs in Lampetia every 24 hours: the Sun disappears for 12 hours, leaving the the Moon and Stars to defend against vast swathes of DArkness for 12 hours until its return. This phenomenon is known as Night and can be observed from Nephele and Adrastea.

Nephele
Nephele, realm of Air and Water, is the next highest plane. It is comprised primarily of air currents and great clouds. The native sapient species here are beings of Air and Water, known as the Fae. With malleable bodies and virtual immortality, they are skilled shapeshifters and talented mages, with a fondness for meddling in human affairs.

Adrastea
Adrastea, world of Earth and Water, is the second lowest plane. It consists of large landmasses and great bodies of water known as oceans. The native sapient species here are beings of Earth and Water, known as Men.

Hekate
Hekate, realm of Earth and Fire, is the deepest plane. It consists of great, subterranean caverns far below Adrastea, in which lakes of magma and jets of fire run. The native sapient species here are beings of Earth and Fire, known as Fiends.