I first heard this song sometime late last year when someone was playing the highest level of this song [Expert 17].
The chart looked really hard, but what stood out to me more was that the song sounded really nice! It is an anniversary special song, and I think back then the song could only be unlocked online (?) I can't really recall, but now it is a song within the original mix.
I did not dare to try the song till much later, and I really enjoy it!
I have since been able to finally clear L17, but the stamina required is quite alot, I still lose from time to time.