"The day I became a Monster" is an original song by rerulili featuring Fukase. This song is an official demonstration song for the VOCALOID4 Library Fukase. This is rerulili's 42nd original work. This song is featured in the Fukase compilation album, VOCALOID Fukase ~THE GREATEST HITS~. This song has entered the Hall of Fame and has exceeded over 100,000 views on YouTube.