This is an add-on to Doom II by Team TNT. There have been three releases, called Eternal Doom I, II, and III. Each release contains all the levels of the previous release, plus some new. Eternal III contains 34 levels. The levels are very large and very detailed (the PWAD is 29MB, containing 15MB of level data, whereas Doom II's level data is only 3MB), and has all kinds of themes. Playing through it takes not only skill ( difficulty is high, yet the number of monsters is not overwhelming), but also time (100% kill demo supplied takes 43 hours) and intelligence (so many switches!). Overall, this is an outstanding add-on, but unfortunately it gets little review, probably because it is released (Nov 1997) so late that most players switch to other FPS games like Quake...
If you can find no way out of certain levels, you may watch the demo, or you can use a WAD editor to see where the switches are and what they do.