In Greek myth, Nyx is the goddess of the night, and gave birth to related gods such as Hypnos, god of sleep, and Thanatos, god of death. Her husband was also her brother, the god Erebus.
Nyx appears in Persona 3 as the game's final boss. The members of SEES manage to defeat the Nyx Avatar, but it still manages to call forth Nyx. The main character uses his Social Link's strength to seal away Nyx, but he sacrifices himself in the process.
During the final battle, Nyx transitions through fourteen forms, each corresponding to a tarot arcana.
Nyx does nothing but taunt the player.
Nyx will only use fire attacks, and absorb all fire damage done.
Nyx will only use ice attacks, and absorb all ice damage done.
Nyx will only use wind attacks, and absorb all wind damage done.
Nyx will only use lightning attacks, and absorb all lightning damage done.
Nyx will only use physical attacks, and blocks all strike damage done.
Nyx will use charm and panic in this form, and will block wind damage done.
Nyx will use physical attacks that hits the entire party. She absorbs all physical damage, and can only be injured by elemental attacks.
Nyx will use all instant death light spells, and will block all fire and ice damage done.
Nyx will use all instant death dark spells, and will also use status effects, such as confusion and poison. Blocks all wind damage done.
Nyx will use any and all elemental attacks in this form. She also absorbs any elemental attack used against her and only take damage from physical attacks.
Nyx will use physical based attacks, similar to Hierophant, but stronger. She also uses Power Charge.
Nyx will use instant death dark attacks, as well as potent fear spells, which will instantly kill the player's party. Somewhat resistant to physical damage, however takes full damage from elemental attacks.
Nyx has access to every previous spell in this form, and takes half damage from everything except almighty. Furthermore, Nyx will use "Moonless gown", which will protect Nyx from all damage done, and reflect any damage attempted. Nearing the end of this phase, Nyx will use his deadliest attacks, the "Night Queen" which does decent elemental damage and a random status effect on all party members. During the Death phase, it one of the status effects that her Night Queen attack may inflict is Charm. If this affects a character with a Persona with healing powers such as Yukari, the charmed character may use a spell usch as Mediarahan on Nyx, thus restoring her to full health.
References to the Snow Queen Quest
Nyx's appearance in Persona 3 is very similar to the appearance of the Night Queen, the final boss of the Snow Queen quest path in the original Persona. For example, Nyx, the Greek goddess of the night, gave birth to Hypnos, Thanatos, and Nemesis. In the Snow Queen quest, the player must defeat the guardians of three towers before they can face the Night Queen; these towers are named Hypnos, Thanatos, and Nemesis, and each contains a guardian that bears the respective god as a Persona. The Night Queen desires to bring forth an Eternal Night, in which the world and everyone in it is forever frozen in ice, while Nyx plans to turn all humans into Shadows. As a more obvious reference, there is Nyx's "Night Queen" attack, which takes its name from the Snow Queen quest's antagonist.