This is when you encounter and fight the Darc Soul version of the character you're playing as. But if you play as many different characters in this area, you'll have to fight all of them during the final boss, so be careful with that... unless you want to do the almost-impossible....

Music copyright me and Bandetta Games

Also, the title is meant to be a question, but files don't support question marks (?), so I went with my traditional double-exclamation-points!!