The eternal dream world of the crystal lies within the Unseen World. Even the gods long to find their way to that place.Fragment entry "Bhunivelze's Sleep" in Final Fantasy XIII-2
The unseen realm (不可視世界, fukashi sekai?, lit. invisible world) is a recurring location in the Fabula Nova Crystallis: Final Fantasy series. It forms one half of the universe, where nothing has form, and is the natural location and source of chaos. Valhalla acts as a stepping stone between it and the visible realm, the land of the living where humans and the fal'Cie exist. The unseen realm is the place where the souls of the dead, both humans and deities, reside. The exact nature and appearance of the unseen realm is not defined, and the location is frequently referred to, but is never seen in the strictest sense, as nothing has physical form there.
The unseen realm resembles depictions of the Void from the rest of the series, the mysterious realm of formation that births crystals that form and sustain planets. Whereas the Void doesn't usually have defined continuity with the normal plane, the barrier between the unseen realm and the normal world in Fabula Nova Crystallis is Etro's gate. "Bhunivelze's Sleep" fragment entry in Final Fantasy XIII-2 describes the unseen realm as containing "the eternal dream world of the crystal".
When the twilight of the gods at last descends upon this world, what emerges from the unseeable expanse beyond that Door will be but music, and that devoid of words: The lamentations of the Goddess Etro, as She sobs Her song of grief.Analect "XIII. Fabula Nova Crystallis" describing the world on the other side of the Door of Souls
After the god Bhunivelze killed his mother Mwynn and took control of the visible world, he became afraid of death, seeing it as Mwynn's curse on the world. He created Pulse, Etro and Lindzei to find the way into the unseen realm to defeat his mother and control all of existence. However, the unseen realm contains chaos, which will destroy any physical object it touches. When Etro, given no powers because of her resemblance to Mwynn, killed herself, she arrived in Valhalla in time to see Mwynn fading into the chaos. Mwynn's last wish was for Etro to protect the balance of life.
During the events of Final Fantasy XIII-2 and Lightning Returns: Final Fantasy XIII, the chaos of the unseen realm breaks into the visible realm, destroying the border between the realms and creating Nova Chrysalia. Eventually, within the centuries, chaos destroys Nova Chrysalia as well, its remains sucked into a newly created unseen realm.
In Final Fantasy XIII: Reminiscence -tracer of memories-, Aoede glimpses the new unseen realm during her near-death experience, mentioning a sea of dark water where the souls fade. After Nova Chrysalia's destruction Caius Ballad and the various incarnations of seeress Paddra Nsu-Yeul reside in the unseen realm to oversee world balance, and shepherd the souls to be either reincarnated, or lie at rest if the soul wishes not to be reborn. Mog stays in the unseen realm as well, guiding Aoede back to the land of the living once she regains consciousness.
Each Crystal wants to birth the Agito: the one who can open Etro's gate to the unseen realm. Arecia Al-Rashia created Class Zero to use the power of a soul with great power to force the gate to open. This becomes known in the legends of Orience as Agito. Gala believes the sacrifice of billions would be a better method, and sends his forces to slaughter everyone when Tempus Finis begins. After they both fail, they reset the world and prepare for the next experiment. The two continue their experiment for 600,104,972 spirals.
Pandæmonium is a temple that appears when Tempus Finis commences and is the symbol and harbinger of the end of the world. It appears to be a portal to "the void" with its final area beyond the depiction of Etro's gate on a platform suspended among "nothingness". As the cadets make their way through the temple to the top they walk into the fiery depiction of Etro's gate to engage the final battle. After walking into the fire, the cadets appear on a platform suspended in a void with nebulae and stars on the background. Whether the fiery image of the gate is truly the gateway to afterlife, or merely a depiction of it, is unknown, but the cadets passing through it to a void-like area alludes to the deities of the world wanting to use human souls to open the gate to the unseen realm.
Arecia is convinced to end the experiment and leave Orience. When she listens to the cadets' souls, she learns they don't want to return for another spiral. Whether either Arecia's or Gala's method would actually work to reveal the gate is unknown, as the spiral revolves over 600 million times without result.