Overworlds (Worlds Collide Supplement)
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About Worlds Collide
Dungeon Master's Guide
Overworld is a catch-all term for all planes and demiplanes that exist "over" the nine worlds of the Material Plane.
The Conceptual Realms
In the universe where the thought, belief, and specualtion becomes real enough to influence the reality itself, the existence of the Conceptual Realms is nothing of surprise. Anything that sentient creatures in the Material Plane, forms itself somewhere in the Conceptual Realms.
(An endless world of desert, where the sand boils and the sun is dim. Old legend of Muhajayd refers the Desert as the afterlife reserved for those who has committed so many crimes that they cannot decide which way to go to atone for their sins. There is only one oasis in the Desert, and that oasis leads to Al-Hadiqasur, the Garden Manor of Bal-Hadad.
(A bottomless, endless ocean that gives birth to the most gigantic monstrosities of all nine worlds. Some say that the Ocean is a sentient world by itself, creating her "children" for some purpose, but nothing is known for sure.)
(The world of underground caverns and bottomless pits that shelters most of the chaotic-aligned fiends. The Pit is right below the bottommost floor of the Spire, and occasionally fiends and lost souls from there fall from the fierce competition to feed the ever-hungry fiends in the Pit.)
(The tower of infinite height that shelters most of the lawful-aligned fiends. Each "floor" is occupied with fiends of the similar ranks, each stabbing each other's back in order to advance to the higher floor. The bottommost floor stands right above the Pit, where the underdogs fall to feed the chaotic-aligned fiends.)