Fayemelina and The One Who Would Be Lord had always been close friends. However, while he was always thirsty for power and embraced evil, she was always in contact with her Light side and had a sense of justice and kindness.
When The One ended the Cosmic Era, he lost his relics, which made up his entire body. He proclaimed himself Lord, but he needed someone he could trust to recover his relics before he could reveal himself to the world.
He chose his loyal friend Fayemelina, but in order to prepare her for the task, he brought out his secret weapon: the Corrupting Root, an item full of unadulterated Lutum which can Corrupt any monster that comes in contact with it. That's how The Lord turned Fayemelina into Fayemalice, stripping her of her Light side. She's lost her kindness and sense of justice, but she's become a top monster with superior power. Fayemalice is purely evil, in debt with The Lord, and ready for her mission.