In February 2004, Gackt released first compilation album The Sixth Day: Single Collection, which was a collection of the singer's previous A-sides, and some being re-recorded. The album was followed three months later by the release of The Seventh Night: Unplugged, another compilation album, containing the acoustic arrangements of previously released songs. He held The Sixth Day & Seventh Night nationwide tour between the releases of the albums.On October 27, he released his eighteenth single "Kimi ni Aitakute",so the unplugged theme would be continued in the release of his fifth studio album, Love Letter on the following year's Valentine's Day. The album was re-recorded in Korean and released on June 16, 2005 for the Korean market. On December 4, he was invited to perform at the M.net/Km Music Video Festival where he was awarded as the "Best Asian Rock Artist".
In 2005, with the release of his twenty-first single "Black Stone" on April 27, Gackt switched to a more alternative rock style, and on September 21 released his sixth studio album Diabolos and had a nationwide tour called Diabolos ~Aien no Shi~ with 36 performances across Japan, which ended with a final performance on December 24, 2005 at the Tokyo Dome.