It’s fitting that, for as dark as Persona 3 is, the final boss of the game is none other than an avatar of Nyx; a malevolent incarnation of the Greek goddess of the night, who, thanks to the heroes getting tricked into helping their unknown enemy for most of the year, end up putting the world in danger by making her arrival possible.

Oh, also, that doofy handsome kid that joined your class partway through the year? Nyx’s avatar given human form. The hows and whys are a bit too complex to really go into here, but suffice it to say, he’s none too happy about his situation once two and two are put together. He even offers the protagonist the chance to kill him without resistance; doing so would prevent Nyx’s arrival, at least for a while, but in the process, everyone would lose their memories of the past year and go on as though the end of the world weren’t the horrifying inevitability that it is.

Of course, that choice nets you the bad ending.

To get the good ending, the protagonist allows Ryoji to live long enough to become Nyx’s avatar. Then, on the night of the next full moon, shit goes down, and the party gathers to fight her. At least, I think it's a her? Ryoji is a guy, but Nyx is a girl, and...uh, hmm. Anyway, Nyx comes prepared, complete with fourteen different phases.

Fourteen. Different. Phases. Each one based on one of the tarot Arcana that are so central to the Social Links and Persona fusion.

And then, once you’ve managed to whittle your way through all fourteen phases (fourteen goddamn phases), Nyx, not the avatar, but the actual Nyx, decides to make an appearance and gets ready to wreck shit. At which point the protagonist says “Nope,” and pulls an Ultra Jesus Attack, sacrificing himself to seal Nyx away.

There’s a lot going on there, but what some Persona 3 fans might not know is that there’s even more to this battle than is truly expressed in this game. Namely, that Persona 3 is, in essence, a remake of the original Persona. Or rather, half of the original Persona.

Yep. Though it was cut from the North American release of the original PS1 version, the first Persona features the Snow Queen quest; an optional path that takes the player on a completely different track from the primary story route. In this route, events transpire that cause the party to become trapped in their high school, which has been transformed into an ice castle. To rescue their teacher, the students must enter three towers and defeat the guardians of each; students that wield the Personae Hypnos, Nemesis, and Thanatos, the mythological offspring of Nyx, and then use a restored mirror to free their teacher from the grasp of a possessed mask. A mask that contains none other than the Night Queen, who is Nyx in all but name.

The similarities don’t end there. Where Nyx in Persona 3 is said to be the harbinger of the Fall, in which everyone is screwed and becomes Shadows, the Night Queen wishes to summon forth an Eternal Night; she’ll freeze the world over, into a barren, icy wasteland in which humanity is exterminated. Heck, Nyx’s final phase in Persona 3 even has a particularly sadistic attack called "Night Queen" which can inflict nasty status effects like Charm.

Also, I should note that it really, really sucks to have a character like Yukari get charmed at that point, as she may turn around and heal Nyx back to full health. (Fuck you, Yukari Takeba. Fuck you. I will never forgive that transgression.)

But really, long story short and all desires of petty, vindictive revenge aside, what it comes down to is that the entirety of Persona 3 and particularly the battle with the Nyx Avatar, is more or less an elaborate remake of that optional quest from the original game. Just with more messianic attributions and a ridiculous number of boss forms.