Oneiris is the World of Dream, and is made up of Light. Unlike Mnemosyne, Oneiris actually has a landscape which can be traversed. However, each individual in Oneiris arrives in a cloud of blindingly white Light. As they assimilate into Oneiris, more of the world becomes visible. It is a world built on the culmination of all of Mundis' dreams; what you see is the sum of all dreams the world has.
Structure & Appearance
Oneiris is, as mentioned, an actual, visible world that people (well, such as they are) live in. It resides in perpetual nightfall, with a black sky and dark blue horizon dotted with bright stars. It has a vast landscape, but one theme is constant: villages. Any settlement made by the Night (natives of Oneiris) is a small village.
The world grows larger as more dreams are gathered, and it has distinct boundaries marked by a black void. As Oneiris grows, the void recedes and more of the world is "revealed."
Oneiris' native inhabitants, the Night, reside peacefully in their cottages throughout the villages of Oneiris. They are formed both from individuals found in peoples' dreams, and individuals who arrive in Oneiris from Mundis. When a person comes to Oneiris, all they can see is white. As the Light begins to take them, they start to see more and more of the actual world. Eventually, it all becomes visible. After a while, the resident Night will begin to notice the foreign entity living in their space. The Night will gather upon the person and consume them, causing them to overflow with Light (and thus become a Night). People who came to Oneiris because they were devoured by Light in the first place usually skip straight to being able to see the world. Soon after their arrival, the Night react.
Relationship with Other Worlds
Oneiris sits on the opposite "side" of the main world as Mnemosyne. It pumps Light into Mundis, and this cannot be changed unless all living things there perish. With every dream dreamt, Oneiris takes the dream and adds it to its own world in exchange for Light energy; it is a natural occurence. The same is true for Mnemosyne and memories.