After perfecting their teleportation devices, the UAC shifted its main focus on the development of a time machine in order to modify the cursed events that their previous action caused to mankind. But as usual, everything went to shit. Very very badly. Some genius thought that travelling through time in order to land during the Earth invasion was a good idea. No one was ever able to close the time connection.
The demons didn't even had a door to break. They had a road all for themselves. After some months, human populations was decimated. after a year, only the 20% of makind was still alive, fighting everyday against the hellspawn. But a lonely man with nothing to lose stood against the demons' horde... the Doom Survivor
To run this use ZDoom.