Ayy, have you ever played Final Fantasy IX? If you haven't, then go and do it! This game is awesome!
..back on track. One of the most awesome tracks from this game is "Dark Messenger", which is boss theme for one of the final bosses. It's long and cool.
So, what this mod does? It replaces Boss Rush music with "Dark Messenger" to feel the challenge!

How to install it?

Well, it's easy! Just take everything you found in the archive to there:
D:/Steam/steamapps/common/The Binding Of Isaac Rebirth/resources

My graditude goes to TYPH3U5, the author of some really cool music mods, like "Malo Mart Music Mod". He wrote a tutorial about how to create them. Link to his profile is here! And link to his tutorial is there!

I hope you enjoy it!