Nyx, also known as Mother Night, is the primordial goddess of the night from Greek mythology. Originally said that she was born from chaos and her name means "the mother night." She is depicted as a goddess draped in long flowing black robes. Versions of her birth and origins vary, but one interpretation has her as a creator goddess, laying a silver egg that produces the gods responsible for the creation of the world. She gave birth to many offspring, including Light (Aether), Day (Hemera), Death (Thanatos), Sleep (Hypnos), Doom (Moros), Retribution (Nemesis) and the Moirae Sisters.
"The goddess of the night in Greek mythology. She was born out of Chaos and gave birth to many offspring, including light (Aether), day (Hemera), death (Thanatos) and the Moirae Sisters among others."
—Shin Megami Tensei III: Nocturne compendium
Nyx is an enemy in Shin Megami Tensei III: Nocturne. She belongs to the Night Race and can be recruited or summoned once the Demi-fiend has reached Level 70 or above. She has strong affinities towards all magic. Another Nyx can also be seen in Ginza, where she owns a lounge. She appears to be a respected and helpful figure who advises the Demi-fiend to enter the Nihilo base located outside of Ginza via forging an alliance with the Mantra and tells him the way of reaching there.
Nyx acts as a boss during the New Moon phase in Shinagawa's ice caves instance. Players can obtain her fusion plugin by giving the red heels drop from the end chest to a Jack Frost NPC in the dungeon's lobby.
Appearing under the name Night Queen, Nyx appears as the true antagonist for the Snow Queen Quest. Her ultimate goal is to create an "eternal night" and uses the Snow Queen Curse as a means of being brought back into the world. When the Demon Mirror is used to free Ms. Saeko, the Snow Queen comes to her senses, causing the now-free Night Queen Persona to show herself. Appearing briefly with her are two unnamed, but recognizable characters from the SEBEC Quest of the game in masks; the Masked Girl indicates she wishes to destroy the school and created the world they are currently in, and the Masked Man humors the girl, but seems to be following his own objectives. The Night Queen merges with the two becoming Queen Asura for the final battle, but is defeated.
Queen Asura is a member of the Tyrant Order in Megami Ibunroku Persona, but her Order was changed to Deity in both the Japanese and English versions of Shin Megami Tensei: Persona. According to the Persona 3 Club Book, the Night Queen can be considered the embodiment Collective Unconscious's awareness of Nyx.
Nyx is explained as the personification of death itself in Persona 3. She is also the final boss in the story, although gameplay-wise the final boss is her avatar. In ancient times Nyx bestowed Death and Night to the world where she is destined to bring The Fall. Nyx, however, lies in hibernation and can only be called by the Appriser of the Fall, which happens to be the incarnation of the Arcana that was never meant to be: Death. When the Kirijo Group was experimenting on Shadows, Mitsuru's grandfather, along with several scientists, knew about the Fall and willingly embraced it. They eventually succeeded in amassing a large amount of Shadows within the experimental facility and rejuvenated Death. However, Eiichiro Takeba declined and destroyed the facilities, causing Death to be forcibly split into thirteen separate Shadows. Defeating the individual parts would cause them to reunite with the 'main' thirteenth Shadow, which was sealed within the protagonist. Death, while within the protagonist's mind, assumed the amnesiac identity of Pharos. Therefore, it is not until the protagonist defeats the remaining twelve Shadows that Death is rejuvenated and reawakened.
Assuming the form of Ryoji Mochizuki, Death was responsible for summoning Nyx and became the harbinger of the Fall. After the protagonist manages to fend off the Avatar of Nyx, the Avatar shrugs the damage away and proceeds to connect to Nyx's true physical body, the moon. As she descends to earth, SEES was rendered immobile, leaving only the protagonist immune to her effect. Through the cheering of SEES and his friends' will to live, the protagonist ascended to the Moon and battled Nyx, where he used his own soul as the Great Seal, sealing Nyx away.
In the Persona 3 Club Book, in a Q&A section it is stated that Nyx is in fact a living being outside reality the size of a celestial body known as "the Planet Eater," that drifted space in a dormant state. Eventually, this being collided with the Earth, and ended up entering Earth's orbit becoming its moon, but leaving its psyche on the surface. This "wave-like psyche" stood a contradiction to the already existing life on earth, which was still at a very primitive state. In order to resist this contradiction, the lifeforms on Earth had a period of evolution and developed a collective subconsciousness in which the lifeforms locked away the psyche of Nyx by suppressing its psyche with their own thoughts. Nyx's physical body entered a healing process after colliding with the Earth from within Earth's moon. By the time Persona 3 takes place, that process was just about done. The Fall is the process in which Nyx's psyche re-enters its physical body, giving Nyx its original form back, meaning a paradox for life on Earth and thus, the end of the world.
The guidebook also makes a quick mention of the Great Will (a recurring power in the main series), stating that gods and demons emerge from humans as another means to defend their psyche from Nyx.
Nyx was briefly mentioned in the playable epilogue of Persona 3 FES, The Answer. It is later revealed that Nyx herself is neither hostile nor malevolent and that she was awakened from the sorrow, depression, and apathy of humankind, believing that humans were tired of living and thus, it is her duty to end their lives. Upon returning to the time where the protagonist seals Nyx, it is revealed that the protagonist is not planning to seal Nyx away from humanity, but Erebus, a manifestation of humanity's negative will and grief from contacting her to bring the Fall.