Eirenus

 is one of the Geniuses of the Fifth Hour and the Genius of Disappointment. It is the primary antagonist of Volume 13, Nightingale's Song.

Synopsis
According to Lelia Mofur and Litka Mofur, Eirenus is targeting Akiba's Adventurers&mdash;a warning that comes true when it easily kills Isaac. It goes on to turn Akiba into a level 65 raid zone and suppresses anyone whose level is over 65 down to level 35.

It is eventually defeated by the members of the Akiba Guild Hall Raid Party.

Abilities
Eirenus' ability lowers the levels of any Adventurer over level 65 down to level 35. However, it has no effect on anyone whose level is 65 or under, and has no effect on Adventurers who are using the Teacher System to lower their levels to 65 or under.

Trivia

 * In the Nuctemeron, where the Genii's names and epithets originate from, Eirenus is the Genius of Idols rather than the Genius of Disappointment.