The World
In conjunction with our world (named in our cosmology, the Tellurian Plane) exists another plane of existence – the Otherworld, or Shadow Plane.
Genesis – the Shift
Eons ago happened a sudden burst of raw, unknown energy throughout the universe – the Shift. It split reality in two planes, the one we live in, the Tellurian Plane, and a duplicate, the Shadow Plane. That Shift released a huge amount of a unique energy, the Effluvium, which was diluted in both worlds, with some areas of heavier concentration.
The two planes
From our point of view the Shadow Plane can be described as an alternate, parallel, plane of existence. When the Shift happened both worlds were absolutely identical. Today, its geography is slightly different from the Tellurian plane, because it went its own way since its inception.
Plant and animal life exists in the Shadow Plane, and the main differences are the survival of many species that disappeared in the Tellurian Plane.
Death happens in the Shadow Plane as well as in the Tellurian Plane. What happens after death remains unknown in both.
The Effluvium
All supernatural elements in both worlds are related to the existence of the Effluvium, the energy that was released during the Shift, and resides mainly in the Otherworld. It is an invisible energy that can pass through any matter, and can concentrate in specific areas. When concentrated, it can be seen as a shimmer in the air that appears to be absorbing and slightly diffracting light.
Crossing between Planes
There are a few different ways to travel between the two Planes.
Portals
Portals are set in a specific position in each world. They are subtle to notice, producing a dark shimmer. People who step within their boundaries will travel to the other portal almost instantaneously. The Portals can be created by creatures who master the Energy such as the Elder Gods, but take a long time to be created and require massive power.
Teleportation
Teleportation is possible but requires a lot of power. Elder Gods can teleport but it takes a toll and they cannot do it often. Humans, generally speaking, cannot, unless they find other ways to harness Effluvium. Objects can be imbued with Effluvium and therefore provide the ability to teleport.
Corrosion
The Corrosion is a corruption that happens when Effluvium is concentrated. It is a type of parasitic, obsidian-like growth that can slowly affect anything.
Living creatures affected by it will have dark, cancerous-like deformations until they die a painful death.
Practical uses of Corrosion
Corrosion can be refined when exposed to heat, in order to have a fraction of it reverted to harmless Effluvium. When fused or absorbed by live beings, the Effluvium can lead to multiple types of affects, mainly:
Sight (visions)
Empowerment (telekinesis)