The Black Egg appears when Kazuomi Hoshina attempts to control Ikuto via his father's violin by infusing it with X-Egg energy, thus creating the black egg. Even though it is made of X-Egg energy, it seems to have some characteristics of a Guardian Egg, indicating it might be more than just X-Egg energy. Ikuto uses it to become "Death Rebel". In chapter 39 of the manga, Ikuto becomes "Seven Seas Treasure". When he talks to Yoru, he states that he did not only Character Transform with him, but with another egg inside himself, indicating a Guardian Character who looks similar to Yoru, except a pirate look.