Erebus

Erebus (エレボス, Erebosu) is the grand manifestation of humanity's grief and negative emotion in Persona 3. A large, dual-faced monstrosity, its very purpose of existence is to contact Nyx in order to bring forth The Fall.

History
In Greek mythology, Erebus was the son of a primordial god, Chaos, and represented the personification of darkness and shadow, which filled in all the corners and crannies of the world. He was the offspring of Chaos alone. He was brother of Nyx and father with her of Aether and Hemera, according to Hesiod (c. 700 BC). According to Hyginus (c. AD 1), he was the father of Geras.

Appearances

 * Persona 3: FES: Final boss (The Answer)
 * Persona 4 Arena: Minor character

Persona 3: FES
Erebus is the final boss of the playable epilogue of Persona 3: FES, titled The Answer.

After reuniting the separated keys needed to travel through time, Aigis decided to go back to the time where the protagonist performs the miracle of sealing Nyx. Metis questions about the protagonist's reasons for doing so, as Nyx herself is not hostile or malevolent. It is revealed that, in actuality, the protagonist was trying to seal Nyx away from Erebus.

Erebus is the manifestation of humanity's grief and negative emotion. By coming in contact with Nyx, it can awaken her, and thus bringing the Fall. Erebus constantly tries to to break the seal, who happens to be the protagonist, now turned into a statue and bound between Nyx and humanity.

However, Erebus's effort has no effect, despite trying to destroy the seal again and again. Noticing SEES's presence, Erebus changed his target to Aigis. Because Aigis now shares the same unique ability as the protagonist, Erebus hopes to use her as a weapon to break the seal.

Not wanting to give up, SEES engages Erebus, but knowing that even they have defeated Erebus, he will reanimate, as long as humanity wishes for the coming of Nyx. Realizing this, the remaining members of SEES swore to help the protagonist bear the burden by making the most out of their own lives. Aigis would later make a vow, hoping to change the world so someday, the protagonist may be released from his bounds.

Persona 4 Arena
Erebus appears again in Persona 4 Arena, fully reformed. Elizabeth, not wanting to do damage to the realm of the collective unconscious, teleports them to the moon and overwhelms it with Thanatos, destroying it with a single strike. Elizabeth laments that Erebus will once again fully reform in a year, and she would have to destroy it again. She then leaves on a journey to find a way to destroy Erebus for good so she can free the Persona 3 protagonist from the great seal.