It's as simple as it sounds, replaces the main menu soundtrack (or adds it to the Main Menu Jukebox mod) with When the Shadows First Lengthened, the main menu theme from Doom: The Dark Ages!

Here's a preview of the song in case you don't know what it is!

Special Thanks To
ZeroZM0 helping me through making both a main menu music mod and making it work with Menu Jukebox.
Smiley, him practically being the only reason I got even HALFWAY done with making any mods. She also makes some pretty baller mods, so go check them out. It's great.
Mick Gordon. Was he involved with this music at all? Nope. But he deserved better than to get underpaid, blamed, and dumped by ID.