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.
Nyx Avatar will keep Moonless Gown up for three turn cycles; the turn in which he activated it, the turn following that, and will drop it after that. This is an excellent time to safely heal, buff, or debuff as he tends to use these turns to buff himself. The third turn of Moonless Gown is also a good opportunity to use Mind or Power Charge as one of his next turns will always be spent lifting Moonless Gown.
You may want to equip a Narcissus Flower accessory as Night Queen has a chance to charm your party members, which can lead to Nyx getting fully healed if the charmed character has Diarahan or Mediarahan. Ailment eliminating items are useful to cure party members without the accessory.
Depending on how many SP recovery items you have on hand and how much damage your party members can deal, the Infinity fusion spell is an excellent skill to use during the Night Queen phase of the battle provided your level is high enough to fuse Vishnu and Ananta.
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 stage is more defensively oriented, using Tetrakarn and Makarakarn to reflect attacks while casting status ailments that are boosted via Foul Breath. It also has powerful Dark magic, but its only means of directly harming the party (outside of statuses, reflected attacks, or instant death) is its Almighty Attack.
This is the Nyx Avatar's true and final form, with four times as much HP as the previous phases. To make matters worse, it automatically halves the damage of all skills apart from Almighty - this is distinct from a standard resistance, and cannot be reduced by a Break spell.
Offensively, this phase uses a variety of attacks, including all of the "-dyne" spells, status buffs/debuffs (including Dekaja and Dekunda), Power Charge and Mind Charge, mass status ailment spells, all four elemental break spells, and the usual Almighty Attack.
At about half health, Nyx Avatar will start using Moonless Gown, which reflects all attacks (including Almighty) for three turns. At 2000 HP, it begins to cast spells according to party members' weaknesses, using the elemental "Break" spells to negate their resistances for more damage during its "Ma-dyne" spells. When its HP drops to critical levels (about 1500), it will begin casting Night Queen, which deals heavy Almighty damage to the party and inflicts a random ailment on each member.
"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)