The Ultimate DOOM was a first-person shooter game created by id Software. It comes with 4 episodes, Knee-Deep in the Dead, The Shores of Hell, Inferno, and Thy Flesh Consumed. It mainly is about hell.
There is a new episode, Thy Flesh Consumed, released with the game. It was NOT released with the original Doom. I think it was because someone like John Romero knew Doom was successful and they wanted to make more money off of it, so The Ultimate Doom, which is exactly the same except for that the fourth episode was added.