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.
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. The unseen realm thus acts as 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.
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.