Sea of Possibilities

Sea of Possibilities (可能性の海) is another world where information exists.

Background
According to Shiramochi-ō while he was a product of the wishes of the Seven Lucky Gods, who's story exist in a book from Ashurbanipal, he is also a being derived from that literary work. Though he admits that the story found in the world is a book that originated as data from a place called the "Sea of ​​Possibilities."

Mass for the Dead Arc
The Sea of Possibilities is revealed to Momonga by Shiramochi-ō who explains his origins. In that, he was the data of the desires from the praises of the gods. While his existence was something that was originally impossible, it was made only possible by a mistake resulting in Shiramochi-ō taking shape in the world. Perhaps because of that, there was an inconvenience because in order for it to manifest, Shiramochi-ō needed to include other "New Year" information and make it a part of his body. If not, he would not exist unless he could develop an area dominated by "event"-like rules.

According to Shiro, after he willingly died at the hands of Momonga through [True Death], it was not the end for him. Instead he found himself back in the Sea of Possibilities, this time with all his memories intact. A year had passed in the New World, which at the time a the celebration ceremony from Nazarick at New Year's attracted the attention of Shiro's creators who then sent another version of Shiramochi-ō to act as a raid boss against Nazarick. Shiro not willing to allow his friend be troubled by his copy, and also unwilling to give up his title as the Ruler of New Year, managed to hitch a ride through the summoning and return to Nazarick, this time though to aid Momonga and his friends.

The power on the other side of the Crack in the Throne Room, unexpectedly activated during Shalltear Bloodfallen guard shift. Using material she had in her possession, such as the book on "Nazarick Academy" the power of the Sea of Possibilities used the data to materialize the grand plan of a school envisioned by Peroroncino. It converted the surface and several rooms in Nazarick into facilities for a school, and the inhabitants of the dungeons had their sense of reality distorted, memories altered to convince them that they were characters such as students and teachers. The only one unaffected by the change was Momonga who called for Narberal Gamma and CZ2128 Delta to assist him to unravel the mystery as they were outside when it happened. After the crisis was resolved and the Great Tomb of Nazarick restore to how it was, after "defeating" the nucleus of the incident, Momonga ordered that no books be allowed near the Crack to prevent another incident. The knowledge that the Sea of Possibilities could bring forth ideas to life was astonishing to him, as it only needed the correct amount of data to make it happen. In his contemplation, Momonga wonders if the power on the other side of the Crack could perhaps make a wish of his into a reality.

It is later revealed that the Sea of Possibilities is not limited to giving desires form, but also fears and imaginations life. This was displayed when a visitor from another world traveled through the Crack attempting to save his lover from being separated. Whilst in the realm, the possibility of being separated manifested in the New World as the deceased enemy of the visitors who aimed to keep the two lovers apart through any means and used hordes of Chaos Beasts as its enactors. The entity was eventually determined to be a fake by Momonga, Demiurge and Albedo. It was then defeated by Nazarick and their new ally.

Shiro who has been inducted into Nazarick has greatly broaden Momonga and his cohorts understanding of the Sea of Possibilities.

Later it is found out that beings born from the Sea of Possibilities are inherently immune to the chaos environment of the Chaos Zone. Beings can not just be born from leftover physical material such as books, but even a minuscule memory can be manifested and be give a life if it’s own. Two examples of this are Enhela Read Gahi and the protagonist, the latter origins causing some concern from his allies.

Furthermore beings born under such circumstances have been inserted into whatever environment they were placed into, to ingratiate themselves in the society they find themselves, the memories those around them where they appear are manipulated, fabricating a false history and allowing these manifestations to insert themselves flawlessly. On the other hand these entities are not aware of their own origins and will play out their roles. Their true origins though are revealed in planted memories that lay dormant in their minds until they cross to the “other side” of the Crack wherein their full memories of their creation will be revealed to themselves.

According to Enhela, he and the protagonist were created for a specific purpose/intent by the Devourer of the Nine Worlds.

Layout
The Sea of Possibilities from one account is where stories exist in the form of data. The power of the sea is quite immense, as it not only can affect the environment, but alter memories and personalities. Such a power even surpasses the immunities of World Item s. Oddly those who are players appear to be immune to the mental effects. Though its influence is not absolute, as a being with enough willpower can override the effects, as displayed by Shalltear Bloodfallen, who managed to reject the power when it forced her to kill Momonga. Her refusal to play her role, caused the reality it created to collapse hence restoring everything as it was.

Trivia

 * From the limited description of the Sea of Possibilities, it bears a canny resemblance to the Akashic records" or "Akashic library", an cosmic compendium of all knowledge and history.
 * In a special crossover event, a being known as Eyepatch Kurumi claimed to be a possibility that was a manifestation of unconscious fear. According to her, she was born from psychic energy of the host body, that leaked out of the world and into an location of what could be called the space between worlds. There "something" allowed her to take substance, giving her possibility form and appearance.
 * According to the background of [Crystal Bride] Evileye, the Crack where the Throne of Kings once stood is a passage that connects the New World and the Sea of Possibilities.
 * After learning about the dangers coming from the Sea of Possibilities, Aura Bella Fiora developed aspirations to be a tank like her creator Bukubukuchagama. Likewise Shalltear Bloodfallen wishes to emulate her creator Peroroncino and be a archer. Mare Bello Fiore followed suit and began imitating his creator. As with Demiurge when dressing as Ulbert Alain Odle. And also Yuri Alpha in her outfit as Yamaiko.