Doom and Doom II have received another add-on in the form Doom Zero, created by Christopher Golden. This add-on can be accessed via a download from the main menu.
Don’t let the name fool you: This ain’t no ‘lite,’ low-calorie version of DOOM. Made in celebration of DOOM II’s 25th anniversary, DOOM Zero contains a whopping 32 levels filled with new enemies, new bosses, new sounds, new music and new sprites.
Despite being faithful to ‘vanilla’ DOOM’s classic gameplay, DOOM Zero still brings its own original ideas to the old-school formula, from branching paths to challenges that’ll get even seasoned Slayers putting on their thinking caps.
Bethesda has a new interview here that has more information on Doom Zero.