Erebus is the main antagonist of the epilogue of Persona 3: FES "The Answer"
Erebus was created by the lurking malice in all of humanity that wishes for death once it fully formed it would call upon Nyx to bring an end to humanity by enacting The Fall.
One Aegis won the Keys from the rest of S.E.E.S they wished to see the miracle the Protagonist preformed that defeated Nyx and allowed humanity to keep on living.
They learn that he had used his own life force as a seal to seal Nyx away from humanity so she couldn't bring about The Fall, however Metis explains that sealing away Nyx is not necessary for she is neither hostile nor malevolent. The truth was the seal was place to keep Nyx from being touched by Erebus the manifestation of human malice the part of everyone that wishes for death and distraction. Yukari explains that when someone's life loses it's meaning the thought of death is actually quite comforting, and Aegis confirms this by saying that during December she too wished to learn death and all the malice from their thoughts helped form Erebus.
When they defeated Erebus it dissolves, however it will reform unless that extremely small malice in all of humanity disappears forever.
Persona 4: Arena
Erebus makes a cameo in Persona 4: Arena being defeated by Elizabeth, she states that by the end of the year it will reform, confirming that it will never go away unless the small part of humanity stops wishing for destruction.