Ereinus, the Disappointment Genius, is a large, dark humanoid giant with antlers and bright linings and a large tank in its chest, which hides the Reverence Genius. The Reverence Genius takes the form of a fetus-like creature.
According to Lelia Mofur and Litka Mofur, Eirenus is targeting Akiba's Adventurers. Upon its arrival to the city, it creates an Instance of a level 65 raid rank dungeon based on Akiba, drawing in all Adventurers as well as the Mofur sisters. Upon its discovery by Isaac, it uses the Teacher System to forcibly lower the level of anyone whose level is over 65 down to level 35 and easily kills the veteran adventurer as a result.
When the Akiba Guild Hall Raid Party reaches the Guild Hall room, Eirenus appears. After a brief skirmish, it uses a mental attack to draw out and shatter the memories that the Raid Party members have with their "subjects of reveration", such as Minori's memories of Shiroe and Touya's memories of Naotsugu. It further devastates the younger Adventurers' morale by taunting them over the fall of their revered ones and telling them to lose hope. The raid is saved when Akatsuki and the Mofur sisters, relatively unaffected and defiant in the face of these mental attacks, rally the younger Adventurers. After Lelia lands a solid blow on Eirenus' chest, a second eponymous Genius is revealed therein: the level 35 Genius of Reverence. Armed with this information, the Akiba Guild Hall Raid Party is able to focus their attacks and destroy Eirenus, turning Akiba back to normal.
Eirenus was able to create a labyrinth-esque Instance resembling Akiba, which pulled in all Adventurers and Ancients residing in the city. Notably, it did not affect ordinary People of the Earth such as Raynesia, who noted that the Adventurers' disappearance from the real city had caused quite a stir. Adventurers in Maihama were also unaffected, indicating that Eirenus' dungeon creation ability is limited in scope.
Eirenus exploits the Teacher System to lower the levels of any Adventurer over level 65 down to level 35. However, it does not effect anyone whose level is 65 or under, nor does it affect Adventurers who are themselves using the Teacher System to lower their levels to 65 or below. Moreover, any Adventurers who were affected by Eirenus' use of the Teacher System were still stuck at level 35 upon their deaths and revival in the regular Akiba's Cathedral, and these Adventurers would be stuck at level 35 until the Genius' defeat at the hands of the Akiba Guild Hall Raid party.
Eirenus also has the ability to sense the subjects of reverence of its targets, such as Shiroe for Minori or Naotsugu for Touya. From there, it draws out and shatters these cherished memories. This mental attack can cripple its targets.
Notably, Eirenus did not use the Words of Death against the Mofur sisters. Whether or not this is because it didn't know the Words of Death or because it didn't recognize them as Ancients is unknown.
- In the Nuctemeron, where the Genii's names and epithets originate from, Eirenus is the Genius of Idols rather than the Genius of Disappointment.
- As Episode 12 reveals, Eirenus is both in the series, with a Genius of Reverence hidden within the Genius of Disappointment.
- In some translations of the Nuctemeron, his name is spelled Eirneus. (The mobile game Chain Chronicle, when its English version existed, opted for this spelling of the same.)
- Encyclopedia of Demons in World Religions and Cultures
- Transcendental Magic by Eliphas Levi, A.E. Waite translation