Erebus was originally thought to be the Mysterious Stranger of the Shadowscythe, but he is in fact something much different and much more sinister.
The Hero first meets Erebus in the Hall of Memories, while he is impersonating a Father of a church in Luminova (now known as Darkovia), under the alias "Seth Cay Dhows" where he has very dark plans with regards to the town. He tends to disregard others as being beneath him, and considers them as inconsequential as birds, as seen from how he talks. Initially thought to only exist as a memory within the Hall of Memories, he turns out to be an entity in the current timeline of Lore. However, at that point in the story he is not a being and thus cannot die, nor can he live - he is immaterial, emulating life like that of a virus. He can control the actions of NightReign just by touching him with his shadow—apparently, this is because he helped NightReign with his transformation.
Later, he uses his powers to attack Falerin. This "kills" Falerin temporarily and The Hero is able to get Falerin to recover, but damage has been done. Erebus makes off with one fragment of Falerin's shattered personality, known as "Ardendor", which leads to the events of Mostly Harmful, wherein Erebus "reveals" more of his backstory: Erebus claims to have been "born" as an aftereffect of a great cataclysm in another universe. He is a transdimensional, transelemental entity. It is revealed that he has done severe damage to Caelestia as well; once altering the timeline so severely that Eldron Larethian, a.k.a. Eldron the "Good" was never born. He is Epsilon (the "man-made god" used by N.O.V.A, an organisation inside the Devourer's network), as well as some other important entity yet to be revealed.
Erebus apparently "shattered" himself at some point in the past, and is collecting his fragments. He also has resurrected the late Atlas Maxwell using one of the Bizarre Flecks and Atlas, is now seeking to advance his position and reconstruct himself by killing the Avatar and collapsing Lore's elements.
Lastly, his name has been mentioned to be only an anagram of convenience. He has also also been known as St. Mohrred of Arturia, an obvious reference to the Character of Mordred or Medraught from Arthurian Legend.
Erebus was sent back to the deepest depths of the Shadow Realm in shadow shackles after The Hero successfully defeated him in The Journey Home. It is, however shown that he has conversed with Drakath, utilizing him to continue advancing his plans, even though he is not physically on Lore.
Powers and Abilities
Erebus has the power to Shapeshift as well being able to use light and dark powers.