The Vale of the Inner Sun is a demiplane within Khyber, a paradise beneath the surface of Eberron. Despite being subterannean, the Vale of the Inner Sun has a sun in the sky, which is fixed in place and never moves. It does, however, change colour periodically. The Vale of the Inner Sun has a peculiar property, in that every creature within it is immortal. Cults of the Dragon Below who seek the Vale believe that it is a blessed place, free of suffering and death. Only one of these is true. It is incredibly difficult (but not impossible) to die in the Vale of the Inner Sun, but this is just as much of a curse as it is a blessing, as the plane does nothing to prevent creatures from suffering the effects of malnutrition or violence, it just stops them from dying. Non-native creature also do not age in the Vale of the Inner Sun, making it impossible for communities to grow.
There is no night in the Vale, but the inner sun does change colour over time, oscillating through all of the colours in the rainbow (and some colours not seen on Eberron at all) at regular intervals, allowing experienced inhabitants of the demiplane to tell the time by the colour of the sun. The demiplane is roughly 100 miles in diameter, but it's possible that there is more to it than the cavern itself.
Creatures that spend an extended period of time within the Vale of the Inner Sun are slowly mutated by the light of the sun, becoming entirely unique aberrations. These entities, some of which have resided within the Vale for centuries, are generally hostile to outsiders and believe that a creature must prove it is worthy to bask in the light. There are portals to the Vale of the Inner Sun beneath Breland, the Labyrinth, the Mror Holds and the Shadow Marches.
The environment of the Vale is wondrous, but the native fauna of the Vale are dangerous, and hostile to those who don't know how to safely interact with them. Floating islands of iridescent rock are home to violent aberrations, and even the birds can draw blood with their songs. The water in the Vale has a psychoactive effect, driving those that drink from it mad with rage. Nevertheless, all of these challenges can be overcome with the right experience, the aberrations can be pacified, the water rendered safe with the right herbs, and those that make their home in the Vale maintain that it is a paradise.
The Jaggeds
In the center of the demiplane, directly beneath the inner sun, a ring of enormous obsidian monoliths surrounds and looms over the central river. These spires are known as the Jaggeds by the inhabitants of the demiplane, and they superficially resemble the claws or teeth of an enormous dragon. A community has built up in the caves under and around the Jaggeds, composed primarily of orcs, humans and half-orcs from the Shadow Marches, and dwarves from the Mror Holds. These people have been in the demiplane for a long time, and many don't remember much of their lives before they came here. They are locked in what they consider to be a sacred test by Khyber to harden them for the end times. The communities that live within and around each spire war with the other spires, but these battles are organised by the community leaders and coordinated. Nobody actually dies, the demiplane prevents it, and combatants are usually healed and ready to fight again the next day.