Revolution!

Revolution! is a megawad made by Thomas van der Velden. It features 32 new levels along with some sprite replacements. It needs a Dehacked patch.

As far as I know, the author never provided a storyline, so here's one I made up: the events take place after Doom II. The evil was defeated and the hellspawn are now hostages of human race. One day, you're transporting some demon captives in a train. Suddenly you hear a loud rumble and computer warns you: "The prisoners are getting away!" "Darn" you say and leave to calm down them a little. But when you arrive, you find out that the country is overriden by hellspawn. It seems that some great creature, called "Forgotten God", is giving the demons power to defeat humans. You must find Forgotten God and kill him!