Trivia
The Master Levels for DOOM II contains a shell program named "DOOM-IT" from which the various new level .wads are loaded. It also features networking/modem setups, automatic recording of level plays ("demo" recordings), and other useful features.
"DOOM-IT" was written by Chris Badger for id Software, and does not appear to be available outside of the Master Levels for DOOM II release.
The Master Levels for DOOM II actually contains 21 maps - "teeth.wad" by Sverre Andre Kvernmo contains a secret level titled "Bad Dream."