Demon Crypt is the ambient track that plays as you explore the crypt dungeon in Hellfire, the map denotes it as "Crypt."
Matt Uelmen originally composed the song for Diablo, but didn't feel it was good enough for the game and it was left out. The song was later resurrected for the Hellfire expansion.
The song can be extracted from the HFMusic.mpq file where it is designated "DLvlF". The WAV file contains meta data with a date as "1997-09-15" and engineer as "Matt Uelmen". It also has a tag for Sound Forge 4.0 which is the assumed software used to record the final version.