Diabolos is the sixth full-length studio album released by Japanese solo artist Gackt on September 21, 2005 in Japan, and on October 26, 2007, in eighteen countries across Europe. Conceptually, it is a sequel entrant to the "Moon Saga" found in the eighth studio album Last Moon and theatre play Mysteries of Yoshitsune I&II, as well prequel to his third and fourth studio albums, Moon and Crescent, the film Moon Child, and novel Moon Child Requiem.

In April 2005, with the release of the album's first single "Black Stone", was the turning point from compilation and unplugged themed albums in 2004 and early 2005, to alternative rock and continuation of "Moon Saga" from the fourth album Crescent in 2003. The name of the album doesn't literally symbolize Latin diabolus, but the darkness in human's heart, which is shown through the characters of the story. The album cover represents a person "taking off the mask", realizing another part of himself inside which is dark and evil.

Conceptually, the story of "Moon Saga" is separated in four chapters, roughly, first chapter from the year 700 to 1300, second from 1300 to 1600, third from 1600 to current time (2000), and fourth from the current time (2000) to near future (2045). The previous work covered the fourth chapter, while Diabolos in between the second and third chapter, roughly between 1500 and 1600. The whole story is set in both Europe and Asia (mostly Japan), depending on the story and meaning wanted to be expressed.

The story from the first chapter follows the "darkside inhabitants" - darts, who were scattered all over Europe, fighting for different countries even against fellow inhabitants in the shadow of history. The second chapter how they tried to make something that will surpass them and for purpose of fighting, so were created 13 "shitos" (使徒, apostle, disciple), but their body was incomplete and couldn't appear under the sun. From the third chapter, now known as "vampires", is followed their life. They didn't pursue their purpose of life, and as become aware of the darkness inside them, raises the question what is now their purpose of existence. After the awakening, two of the thirteen shito died (the man and woman shown in the music video of "Todokanai Ai to Shitteita noni, Osaekirezuni Aishitsuduketa..."), leaving behind eleven. Five out of the eleven decided to stand up and fight against the darts, their creators. They wanted to prove that there was meaning in their existence, however they were outnumbered, and there was only one shito survivor.

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