Ultimate Doom takes the original 3 episode Doom game and revamps it with a whole new episode and graphical updates. The new fourth episode - Thy Flesh Consumed, is designed for Doom experts. The levels are long, non-linear, and full of the hardest demons of all. Consider yourself a pro if you can beat this on any difficulty higher than Hurt Me Plenty.
The internet multiplayer side of this game has also been updated and runs smoother than before. There is less lag time and more options for improved fun. Of course, you can still test your skills on the original episodes, which provide a good challenge on the Nightmare setting.