Death Arcana, also known as Nyx Avatar is the final villain in Shin Megami Tensei: Persona 3, acting as the game's final boss. It is the physical incarnation of Nyx, the Night Mother, and arrives on Earth to usher in Death and worldwide extinction. It is formed partially from the body of Ryoji Mochizuki, who incubated the Avatar until Judgment Day.
After the defeat of the twelve Full Moon Shadows, Ryoji Mochizuki is sent from Erebus to act as the Appriser of the Fall. After Ryoji accepts that humanity's wish is for Nyx to eradicate them, he gives the members of SEES the choice to either kill him and remain ignorant of their impending demise, or spare him and face imminent death. After telling them, Ryoji left, not before telling them "looking forward for the next year", presumably himself hoping that they could stop Nyx as well.
Should the team spare Ryoji's life, he leaves them, and in doing so fuses with Nyx to become the Nyx Avatar.
In January 31, Ryoji appears in the top of Tartarus, where he now serves as an avatar of Nyx, bearing the potential and skills that the twelve previous shadows possessed. Throughout the battle he shifts from one arcana to the other, while explaining their motifs and purpose, and upon shifting to his final and true arcana, Death, his monotonous voice finally seams through a melancholic tone. While arming an attack, he tells the protagonist not to regret, as this is the protagonist's "own decision".
After the protagonist manages to fend off the Avatar of Nyx, Ryoji laments that should more people have been like S.E.E.S, then perhaps the Fall could have been avoided, and shrugs off the S.E.E.S' assault, and continues his summoning of Nyx. As S.E.E.S fall into despair, the protagonist rises upon the moon to face Nyx. Through the cheering of others, he manages to perform the Great Seal, and thus sealing Nyx. Ryoji's voice would later reappear, telling that the protagonist had already found his answer to life, and fades away.
It's stated in "The Answer" by Metis that Nyx is neither hostile nor malevolent that it is answering the small voices of malice in all humanity that wish for The Fall.
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. Death's true form actually identical with Protagonist's Thanatos persona, as the said Persona mainly represent his friendship with Death back when still in his younger human form Pharos. As Ryoji Mochizuki, he looked like teenage version of his previous incarnation Pharos, which showed that he now had become a complete entity.
In battle, he harnesses a total of 14 Arcanas - from the Fool to its true Arcana, Death. He changes his Arcana using its unique skill Arcana Shift, and each time his 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 a myriad of random status effects.
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. This is true to his purpose to bring the Fall.