No one has ever heard the cry of Kereberun, a Heyshuren (a hybrid of humans between Hey and prehistoric animals, without gender distinctions, and particularly strongest among the strong). However, his roar is said to be as powerful as thunder, capable of bringing death to living beings. But Rubal, who became the physical embodiment of Morgan out of the desire to fight Haken, or Rebellu, found Kereberun with mystical abilities. Through an unequal contract, Rubal bound Kereberun to himself to guard the fortress of Weidete. Despite being tragically bound due to his innate abilities, Kereberun always appears sombre. However, it's said that he reveals mysterious methods by which humans, who play beautiful music, can gain strength by adhering to the laws of nature.