Realm of Light
This is Eradu, the world of Man, the seat of Haven the Perfect City, a place of Order. While things outside the Dome of Haven are a terrible and dangerous mess, this is the prime dimension, the material plane. Eradu and her moons were created by the World God Xin in ancient times and the current state of things is because of the vile Phoenix Legion, who slew the World God but failed to bring about the end of Eradu itself.
Another dimension of being that lies right next to the Realm of Light. It pushes through and grows like a fungus or mold, corrupting and twisting things. These tears in reality lead to the Underworld itself, a place of madness and perverted order. These "dungeons" are part of a greater problem.
The Astral Plane
The Astral surrounds the Realm of Light and the Underworld and, many suspect, all places. Most things have a form in the astral, people as shadows of themselves, sleeping wrapped in their silver cords. Wizards can step into the aether of the Astral, a gift or curse of their connection to the Patchwork Kingdom. From the Astral a wizard could travel to any other dimension.
A filthy shadow cast by the Realm of Light cast upon the Astral. It is a place of darkness and blood. The gloom occasionally creeps into the foulest places of the Realm of Light - most often the residue of a wizard travelling there. Wizards attempting to see into the Astral often 'miscast' themselves into the Gloom. The reasons for this are unknown, the outcomes are often unfortunate.
The Patchwork Kingdom
Wizards have a connection to the Patchwork Kingdom - a hole in their soul - that allows the power and chaos of this otherworldly realm to be channeled and controlled - at least up to a point. It is a rarely glimpsed place of unrelenting chaos; pure energy of form and thought; demonic nightmares and creatures strange beyond comprehension. It is a place of demons.
Reference of the Fairy Courts, Lands of the Fey (also Faerie, fae, fyrae, and all manner of spellings have been found) are common in the tales of children, drunk, and adventurer alike. Elves of all manner, hobgoblin, bogarts, and bogeymen run amok, beautiful people offering delicious food and unexpected wine, and a war so ancient that everything is said to be a pawn. If the five courts (Summer, Winter, Spring, SUmmer, and Moonlight) even exist, be wary if things seem too idyllic.
Somewhere beyond Erau, not exactly in the Realm of Light, hiding in the darkness between the infinite stars, are the Star Realms. The Old Gods, if they exist or ever existed, wait here sleeping. Their avatars can sometimes be seen searching the night skies, looking for worshipers, casting their strange repeating melodic messages. Members of the Esoteric Society claim that in the most ancient texts Eradu was part of a vast network and the Star Realms were but a moment away.
Adventurers sometimes run across rumors of a place called Sigil, a city in the heavens at the center of the universe. They also find references to the Variance, some sort of subtle shifts in Realm of Light and perhaps elsewhere. Talk of divergent streams and parallel lines, the appearance of odd mathematics and references to the Machine Core; these are signs of Sigil and her three rulers: The Lady of Pain, The Lady of Suffering, and the Lady of Dreams.