Eirenus, also called Eirneus or Eirenusu, is a high-ranking commander within the Black Army, serving directly under the Black King and bearing the title "Demon of Destruction." She possesses lilac hair, red eyes, and distinctive purple skin, wearing minimal armor that covers only essential areas. A tattoo marked with the number three is visible, its meaning undefined.

Originally an adversary of the Black Army fighting alongside allies, Eirenus suffered defeat and trauma that triggered her "Blackening," transforming her into a demon and fundamentally altering her allegiance and personality. She is now confrontational, frequently targeting Phoena specifically and employing psychological manipulation against opponents.

Functioning as a magician-class unit, Eirenus specializes in area-of-effect attacks and battlefield control. Her key skills include "Ancient Nova," causing moderate damage and knockback to all enemies in her row, and the powerful super skill "World Destruction," dealing widespread damage, reducing enemy defense, and causing knockback. Her abilities enhance combat: "Crimson World" enables ranged attacks to pierce enemies, inflict fire damage, and restore health to demon allies based on damage dealt, while "Black Amplifier" boosts her attack, defense, and critical hit rate with accumulated mana. Her mere presence passively increases attack and defense for all demon allies.

Eirenus acts as the Black King's direct enforcer, often spearheading the Black Army's early assaults against the Volunteer Army. Her prominence wanes as greater threats emerge later in the conflict. Narrative hints suggest her past identity and trauma fuel her current antagonism, though explicit details regarding her origins or potential redemption remain unexplored.