This expansion for Doom II is only available through Doom Classic Complete, which I highly recommend. The Doom series was a popular series in the 90s and has had another game recently: Doom II. The expansion pack consists of 20 "WADs", with one WAD containing a secret level [the first screenshot on the store page]. Think of this as a bonus, as steam has yet to sell this separately.
This game runs via the acclaimed emulator DOSBOX, and you will find a copy of the application in the Steam directory. You will be able to select WADs via the Doom-It interface, where you can select one of the 20 levels. The WADs are extremely well designed and my personal favourites are TEETH and MANOR. However, this game is certainly not for everyone.
Try the Doom I shareware on DOSGames using DOSBox before you make a purchase. If you don't like the limitations from the original DOS version, you can copy the WADs into a source port's directory [such as ZDoom and PrBoomPlus] and you can play it from there. They introduce new High Definition graphics and add jumping and aiming abilities.
I cannot give this a score as it is not identifiable as a standalone title; you will need Doom II to play this so that is why Steam doesn't sell it separately. Overall, this is a great modification for Doom II. If you want to use other mods on Doom 2, you will need to use this command on DOSBox Doom2.exe -file [Mod].wad.