Nyx Avatar is the final boss of Persona 3, as it is the final enemy actually fought by SEES, instead of being a scripted battle. Before becoming the Avatar it was known as Death, the harbinger of the Fall and incarnation of the Arcana that was never meant to be: Death. Because it went through many incarnations and separations, Nyx can be recognized as any one of the following entities: Death, Ryoji Mochizuki, and Pharos.
In battle, it harnesses a total of 14 Arcana - from the Fool to its true Arcana, Death. It changes its Arcana using its unique skill Arcana Shift, and each time its elemental affinities and used attacks change. At his final phase, he uses two unique skills - Moonless Gown and Night Queen. The former temporarily renders all attacks to be reflected against the attacker, including Almighty attacks such as Megidolaon. The latter deals heavy damage to all opponents, while inflicting myriad random status effects. Nyx Avatar is known to bring about the Fall which means to end the world.
Like the Shadow Arcana, the Nyx Avatar represents the inverse interpretation of the Death Arcana. While the Death Arcana represents regeneration, deep change, and cycles, the inverse Death Arcana's meaning is purely death, doom, and an end to something.
This boss can attack twice each turn (more if its attacks strike critically or a weakness). This boss has 14 different stages, and with the first 13 having 1500HP each and the last having 6000HP, the boss has a total of 25500HP.
The only move the boss has throughout all its stages, except the last, is Arcana Shift; when the boss's HP is depleted in that stage, it will automatically (regardless of turn order) use this to switch to the next stage.
Like every boss in both this game and Persona 4, Light/Dark moves are blocked.
During this stage, Nyx will not choose to attack, instead simply "smiling arrogantly" until this stage's HP is depleted. Because of this, it is largely recommended that the player ONLY use Rush to conserve SP/HP and save time.
This stage tends to use its 2 turns to cast Power Charge, followed by any of its strong physical attacks. Its immunity to physical attacks protect it against Tetrakarn/Attack Mirrors as well. It also still knows Almighty moves, but you can use a Slash/Strike resistant Persona to resist those moves.
This phase knows all -dynes spells, stat decrease spells, charges, spells that inflict panic, poison, and fear to all enemies, elemental break spells, and Moonless Gown, which reflects all attacks and Night Queen, which deals high Almighty damage and can inflict random ailments.
"The moment man devoured the fruit of knowledge, he sealed his fate..."
"Entrusting his future to the cards, man clings to a dim hope."
"Yes, the Arcana is the means by which all is revealed..."
"Attaining one's dream requires a stern will and unfailing determination." (switching to Magician Arcana)
"The silent voice within one's heart whispers the most profound wisdom." (switching to Priestess Arcana)
"Celebrate life's grandeur ... Its brilliance ... its magnificence..." (switching to Empress Arcana)
"Only courage in the face of doubt can lead one to the answer." (switching to Emperor Arcana)
"It is indeed a precious gift to understand the forces that guide oneself..." (switching to Hierophant Arcana)
"There is both joy and wonder in coming to understand another." (switching to Lovers Arcana)
"One of life's greatest blessings is the freedom to pursue one's goals." (switching to Chariot Arcana)
"To find the one true path, one must seek guidance amidst uncertainty..." (switching to Justice Arcana)
"It requires great courage to look at oneself honestly, and forge one's own path..." (switching to Hermit Arcana)
"Alongside time exists fate, the bearer of cruelty." (switching to Fortune Arcana)
"Only with strength can one endure suffering and torment." (switching to Strength Arcana)
"In the face of disaster lies opportunity for renewal." (switching to Hanged Man Arcana)
"The moment man devoured the fruit of knowledge, he sealed his fate ... Entrusting his future to the cards, man clings to a dim hope. Yet, the Arcana is the means by which all is revealed. Beyond the beaten path lies the absolute end. It matters not who you are ... Death awaits you." (switching to Death Arcana)
"You cannot avoid Death. To live is to die; they are one in the same..." (before first Moonless Gown activation)
"The Arcana is the means by which all is revealed..." (before switching Arcana)
Though using Ryoji Mochizuki as its Avatar, SEES still refers to Nyx and its Avatar as "She" in both "The Journey" and "The Answer".
Unlike most final bosses in the series the party still gains XP after defeating Nyx Avatar, this was either an oversight on the developers part or intentional due to the scripted battle with Nyx that happens afterwards.