Heaven exists outside of reality, and is the place where all of existence began. It is incapable of being accessed by any normal means, and is the source of magic in most universes within Xevv. Visually, to those who can see within it, it's like swimming in a giant pool of golden milk. Gods physically manifest here, and decide causality in this place.

Lithium Info Page

A strange dimension. The beings of the God Cage are the most powerful beings in all of existence, capable of destroying reality itself if they thought it best.

The God Cage itself is a finite dimension, possibly the first of any. While it cannot be visually described, it is known that all particles in this dimension are made of pure, primordial magic of which is untouchable by any outside source; hence the name, a caged dimension for only the Gods to roam.

Hermes is the only known God capable of holding a physical form outside of the God Cage, and moreover the only one capable of outside communication. His demise during the events of the calamity have halted all further knowledge about the dimension, leaving it merely the mental shape of a huge chrome cylinder box.

Trivia

• The info page for Heaven in Lithium describes that Gods cannot hold physical form outside of Heaven. This is not entirely true; while they are capable of holding physical form, they do not generally wish to, as they must pass through Flux to get to reality, and do not have any purpose being there. It is also a large risk to manifest in reality, as being touched by (or touching) a God will grant one temporary, infinite metaphysical strength, possibly destroying all of reality. Instead, they usually use causality to grant them a body that is not theirs to correct things.
• The line "leaving it merely the mental shape of a huge chrome cylinder box" is a reference to a Venetian Snares album named Huge Chrome Cylinder Box Unfolding.