This is based off of a song called "U.N. Owen was her" Flandre Scarlet's theme from a game called Embodiment of the Scarlet Devil - AKA Touhou 6. It is a bullet hell series where you play as a heroine and try to get trough all 6 stages and then if you choose, to fight the Extra stage boss. In Touhou 6 Gensokyo (where they live) is shrouded by red mist by a vampire known as Remilia Scarlet and you have to go to her mansion to defeat her and stop the mist.