A terrorist organization founded and led by Lazarus and Marcellus, Ilias Kreuz interprets Goddess Ilias’s teachings very seriously; they will show no mercy towards monsters and will attack them no matter the cost. Aiming to exterminate all monsters, these humans organize and assault their settlements. Their base of operations is located at Gold Port's church.
Originally planning to spread the previous Monster Lord Alice XV (Fifteenth)'s wishes of coexistence between humans and monsters after she sacrificed herself to the humans, a young Alice XVI (Sixteenth) went berserk after misinterpreting the events upon seeing her mother's deceased body and murdered Karen, a woman whom both Lazarus and Marcellus loved. Out of pure vengeance, they formed the terrorist organization and had forsaken Alice XV's ideals. Alice XVI would later regret sparking these chains of events.
Sometime after, Marcellus met and married Lucifina and fathered a son, Luka. Eventually realizing that Karen would not have wanted revenge, Marcellus went on to betray the organization's ideals and began to aid monsters, hiding them in Enrika. However, Lazarus eventually found out and, believing that his betrayal also meant betraying Karen's memory, attacked his best friend out of anger. Having expected Marcellus to evade, he had not seen the attack due to an injury from Alice XVI that left him partially blind, and quickly succumbs to the injury. The death of his best friend would drive Lazarus into grief and despair; he would then lie about Marcellus's death being caused by monsters and continue enforcing the organization.
Marcellus's involvement with the organization had adversely affected his son Luka, who believed that his father was a crazed terrorist. Luka planned on becoming a martyr and would rather die by to correct the sins of his father instead of losing his way. However, he was eventually convinced by Alice XVI, who had become his traveling partner, that the sins of the father do not extend to the child.
In present day, Lazarus is seen attacking a mermaid school in Port Natalia and a mermaid pub in Sabasa, and steals a Garuda Girl egg from the Bird God Shrine with the intent to destroy it. A ranking member named Lupton sulks around the shrine and Luka nearly strangles him to death for lying about killing monsters, who then reveals that Lazarus took the egg back to the organization's headquarters. Luka confronts him, telling the guards that Marcellus's son wishes to see him, and a heated argument sparks between the two where Lazarus reveals he was Marcellus's killer and explains the origin of the founding of Ilias Kreuz and vehemently denies the possibility of coexistence. Alice comes in and it only takes a minute for him to recognize her; she explains her sorrow and regret, but he isn't listening from the monster who murdered his companions - in his eyes, anybody who lies down with a monster is betraying their memory, and that includes Luka’s father. Luka finally snaps and threatens to kill Lazarus with his Iron Sword before being stopped by Alice; rather than letting Lazarus die as a martyr, she turns him to stone. The two guardsmen walk in and flee on the sight of their petrified leader; with that, Ilias Kreuz is dissolved and leaderless.
Lazarus later recovers from petrification just as Ilias had forsaken humanity after being rejected by Luka, with the intent to purge humanity and having her forces attack all human settlements including Gold Port. Coming to a resolve with his memory of Marcellus, Lazarus sacrifices himself to protect the town. This sacrifice unifies the former members of Ilias Kreuz to unite against Ilias and ally with the monsters. Once Ilias is defeated, the former members swear not to reform the terrorist organization.