Caelistis (3.5e Campaign Setting)/Cosmology
Cosmology and The Planes
The Planar Cosmology of Caelistis is surprisingly limited. While some common planes exist, such as elemental planes, the ethereal, and the shadow plane, there exist very few others. The elemental planes were believed to have been made by the Greatest Four as a sort of collection of raw materials from which they could build anything, while the shadow and ethereal planes were believed to have existed ever since the creation/discovery of Caelistis, being bi-products of the creation of the Material Plane.
The Greatest Four, (Io, Kelsh, Amannar, and Ebnaya) were believed to have their own planes, where they kept their most potent creations. It is also believed that from these planes come all creatures summoned by magic, aside from those exclusively from other planes, such as elementals. Travel to the planes of the Greatest Four is believed to be impossible, but the strongest of magicians could possibly find a way. They are blocked by divine will, but as magic is made from divine spark, a strong enough spark could feasibly break through. If someone attempts to travel to any of the planes of the Greatest Four, a DC 40 caster level check it required to get past the divine barrier.