Nyx Avatar is the final boss of Persona 3, fought by SEES. Before becoming the Avatar it was known as Death (or "Thanatos, the bringer of death"), The Appriser, and the harbinger of the Fall. Death is the incarnation of the Arcana that was never meant to be, and because it went through so many changes, Nyx can be recognized as any one of the following entities: Death (Thanatos), Ryoji Mochizuki, Pharos, and Nyx Avatar.
In battle, it harnesses a total of 14 Arcana - from the Fool to its true Arcana, Death. It shifts its Arcana using its unique skill, Arcana Shift, changing its elemental affinities and attacks. During the final phase, it uses two unique skills - Moonless Gown and Night Queen. Moonless Gown reflects all attacks, including Almighty skills. Night Queen deals heavy damage to all opponents, inflicting a myriad of random status effects on the party.
Like the Shadow Arcana, Nyx Avatar represents the inverse interpretation of the Death Arcana. While the Death Arcana represents transition, regeneration, deep change, and cycles, the inverse Death Arcana represents doom, destruction, and an end to everything.
Nyx Avatar can attack twice each turn (more if its attacks exploit a weakness or strike for critical). The fight consists of 14 different phases, based on its Arcana. Each Arcana has 1,500 HP each, except for the final Death Arcana which has 6,000 HP. In total, the boss has 25,500 HP. Arcana Shift is used in between each phase (regardless of turn order), after its current HP pool has been depleted.
During the final phase, Nyx Avatar will cast Moonless Gown, which lasts for three turn cycles; the turn in which it was cast, the following turn, and will drop it after that. During this time, the boss tends to buff itself, giving the party a chance to safely heal, buff, or debuff, as well. The third turn of Moonless Gown is also a good opportunity to use Mind or Power Charge.
Night Queen has a chance to Charm party members, which may lead to them casting a full-heal spell on the boss. Equipping a Narcissus Flower accessory will resist Charm, and ailment-eliminating items are useful to cure afflicted party members. Using the Infinity fusion spell (Vishnu and Ananta) during this phase will ensure no party member becomes Charmed, as it renders the entire party invulnerable for one full turn.
As with every boss in the game, Light and Dark attacks 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. Immunity to physical attacks protect it against Tetrakarn/Attack Mirrors. Using a Slash/Strike resistant Persona is a good defense against its Almighty Attack.
This Arcana is defensive-oriented, using Tetrakarn and Makarakarn to reflect attacks and casting status ailments that are boosted via Foul Breath. It attacks using powerful Dark magic and Almighty Attack.
This is the Nyx Avatar's true and final form, with four times as much HP as the previous phases. It automatically halves the damage of all skills, apart from Almighty - this is distinct from a standard resistance, and therefore 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 from 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)
"Let us finish this. It is the path of your choosing." (Before first Night Queen Activation)
"The Arcana is the means by which all is revealed..." (before switching Arcana)