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.
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, Bhunivelze does not know that 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, Nova Chrysalia is destroyed with 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 appears to stay 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's reasons for creating Class Zero is 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, Arecia resets the world and prepares 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 start the final battles. After walking into the fire, the cadets appear on a platform suspended in a void with clouds 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.
Eventually, Arecia is convinced to end the experiment and leave Orience forever. When she listens to the cadets' souls, she learns they don't want to be revived 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.