This is my version of a mod that switches The Binding of Isaac's soundtrack with various songs from Undertale.

I know there are a few other mods that do this, but I didn't want to feel like I was fighting Undyne the entire time I'm in Sheol. All music belongs to Toby Fox, yada yada yada. Here's a partial list of the music I changed:

Basement -> Ruins
Cellar -> Uwa!! So HEATS!!♫
Burning Basement -> Another Medium
Boss fight -> Enemy Approaching

The Caves -> Waterfall
The Catacombs -> Snowy
Flooded Caves -> Quiet Water

The Depths -> It's Raining Somewhere Else
Necropolis -> Mysterious Place
Dank Depths -> An Ending
Mom -> Spear of Justice (No, I didn't use Heartache for the Mom fight. Toriel really doesn't want to kill you, while Isaac's Mom really does want to kill him. According to Toby Fox, Undyne was "the first character who was really going to kill you" so it made sense to use Spear of Justice as Mom's theme.)

The Womb -> Here We Are
Utero -> Here We Are
Scarred Womb -> Here We Are
Mom's Heart -> Part 1 of Your Best Nightmare (Think about it: this is a previously killed being, a different part of it suddenly coming back to life and trying to kill you again while looking gross doing it. Sounds like Photoshop Flowey, to be honest.)

Blue Womb -> Barrier
Hush -> ASGORE (One of Hush's attacks, the slowly growing circle of tears with one gap in it, reminds me of one of Asgore's attacks of the same style.)

Cathedral -> Respite
Isaac -> Heartache
Sheol -> Undyne
Satan -> You Idiot (You idiot! You're a tiny child and you're trying to battle Satan himself. This track is also used as the theme for the Deal with the Devil room.)

The Chest -> Don't Give Up
Blue Baby -> Stronger Monsters (Nothing else fit)
Dark Room -> "But nobody came" (Also known as mus_toomuch, it's the creepy music that plays on the Genocide route after a "But nobody came" encounter occurs.)
The Lamb -> A different section of Your Best Nightmare (I'm serious, Flowey's music can fit so many parts of this game!)

Void -> Memory (You may wonder why I chose such a slow, peaceful song for a floor as chaotic as The Void. But think about it, you keep seing rooms and enemies you've encountered before, almost like a floor full of memories. Trust me, the longer you're stuck on the Void floor with this music on, the more it makes sense.)
Delirium -> Amalgam (Pretty self explanatory: Delirium is all white, changes shape, looks goopy, and it really hard to kill, like the Amalgamates.)

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Want a more-likely-to-be-updated version? Check out the mod on Steam! https://steamcommunity.com/sharedfiles/filedetails/?id=1319705896