Universe Arcana

The Universe Arcana, is a powerful arcana in the Persona series. The Universe Arcana is also commonly associated with Social Links, where its power is based upon the Protagonist's bonds with others. Much like the Aeon Arcana, the Universe is not part of the Rider-Waite Tarot Deck like the others, but rather from the Thoth Deck, numbered XXI, replacing The World Arcana. The first and only known user of The Universe is the Protagonist of Persona 3.

Persona 3
The Universe Arcana serves as catalyst of sealing Nyx in Persona 3. Knowing that Nyx cannot be defeated, the Protagonist was taken to the Velvet Room. Igor comforted the Protagonist by informing him that he is, in fact, still alive. He questioned the Protagonist if he could hear the sound of his friends cheering him to not give up.

Through the voices and cheering of the others, Igor crafted the Universe arcana, and promptly gave it to the Protagonist. Igor remarked that this is the first time he has seen such a powerful arcana, and tells the Protagonist the Universe arcana symbolizes the end, or a new beginning. Awakening in the highest plane of Tartarus, where Nyx descends, the Protagonist ascended towards Nyx' true body, the moon, in attempt to stop the commence of the Fall.

However, even with the Universe Arcana, the Protagonist was still too weak to unleash its power, due to Nyx' assault. Through the cheering of SEES and others, the Protagonist managed to conjure enough energy, and performs the Great Seal, sealing Nyx away from humanity. However, in return, to use such tremendous powers, the Protagonist sacrificed himself, using his life's essence to seal Nyx away.

In the playable epilogue Persona 3: FES, titled The Answer, it is revealed that that the Protagonist is not directly sealing Nyx, as she is not herself a malevolent entity. He instead exists as a barrier between himself and Erebus, the embodiment of all the world's sorrow; in essence, the secret wish in the hearts of all to seek death. If Erebus touches Nyx, she will destroy the world. Erebus constantly attacks the seal though he remains barred from an enormous golden gate that the Protagonist' soul is guarding.