Muu was a extremely powerful monsteras the in the of ancient times. His dark powers eventually grew too powerful to be , and he created legions with his ability to transform good monsters into his evil for of the world. As a counter weapon, the people of Pangea created a second monster, the Phoenix, to defeat him. Both were evenly matched. The people of Pangea then came up with a way to defeat Muu. They used a special machine to separate his spirit from his body. An unfortunate side effect was it did the same to the Phoenix. It also cleared all of the other monsters' memories and locked them in mystery discs.
Muu's soul became a Mysterious Disk imprisoned in his castle, along with the Mystery Disks containing many of his old troops. Centuries later, after he was banished from his village, Holly's father went inPhoenix without the aid of the Magic Stone, but found Muu's castle instead. When he unintentionally unlocked Muu's soul, it entered his body and thus Muu was reborn. He reassembled his army with his Big Bad Four enforcing his injustice, terrorized the world while in search of the location of his old body.
With help from his "daughter's" Magic Stone, Muu found his body buried in the north and after it was excavated, used a special machine to merge into it. But the fusion was not complete and so he sent his general Durahan to deal with the heroes while he used the Magic Stone to completely restore his body and started the search of the Phoenix's soulless body so he could. Eventually, the "true Muu" emerges and resumes his 'destroy the world' plan. The Phoenix was locked within the Spirits of Mochi and the other monsters. Only in the end, due to Holly's father regaining some control, they destroyed each other so the cycle will not continue...
But Muu's Monster Disc was uncovered and became the goal of Genki and his reunited gang to find it and separate Holly's father from Muu so it can be sealed permanently. However, while they accomplished their goal, the Monster Disc that held Muu was taken by Durahan who intended to absorb Muu into his being so he can. But the monster overpowered him and devoured his minions in the process. Genki eventually managed to free Durahan and his group from Muu, stopping the monster for good.