Doom II: Hell on Earth is the hit sequel to Doom, one of the most influential first person shooters of all time. The Doomguy has returned from his mission on Mars and his banishment to Hell, only to find that Earth has been invaded by demons. He must now kill all the demons on Earth and go back to Hell so he can kill everything there.
and Zero Master
utilize a variety of glitches and tricks that exploit the Doom engine's quirks to complete the game on the "Ultra-Violence" difficulty as fast as possible. The RNG was also manipulated to ensure the enemy AI acts as the runners' intended. You can read the authors' notes
for more details on how each level was achieved and a list of all the glitches used.