What is THIS?? [Hiiro no Fuusha - Sound Horizon] First time REACTION!

Hey everyone! I had this link saved for a long time already, so I have no idea anymore in what context this was recommended xP But since it was recommended and I am kind of trying to catch up with your suggestions, why not check it out! Link to original video:    • 14 Hiiro no Fuusha | Sound Horizon | Live ...