The Ultimate Doom
The Ultimate Doom (or The Ultimate DOOM as a brand, and informally "Ultimate Doom") is an expanded version of Doom released on April 30, 1995, that adds a fourth nine-level episode to the game, Thy Flesh Consumed. The enhanced version was made as an incentive in the distribution of the boxed game through retail stores and venues, although to be fair to fans who had previously registered Doom, id Software provided them with a freely available patch to upgrade their copy of Doom version 1.9 to The Ultimate Doom.
The expansion's design was led by John Romero with American McGee and Shawn Green. Romero recruited two prominent level designers from the fan community to complete the team: John Anderson (Dr. Sleep), who would later help John Romero with Daikatana; and Tim Willits, who eventually became id Software's lead designer. A few extra new graphic assets were created by the artists.
Ultimate Doom was produced as a retail version of Doom while id Software was involved in other projects.  John Carmack has explained that it was always a secondary focus spearheaded by GT Interactive.
The plot of the new episode is indicated to occur between the events of the original game and Doom II. Though the exact location of the new levels is unknown, the ending of episode 3 implies that the protagonist is on Earth after entering the hidden doorway from Hell. The ending of the new episode declares that the spiderdemon had already sent forth its legions, and that the player is aware of them rampaging through Earth's cities.
As Doom II features were present in the executable, some of these, namely sector type 17, the key-requiring switches, and other linedef actions such as blazing doors were used in the additional levels.
In addition to making two of the new levels, John Romero modified the first level of his first episode, Knee-Deep in the Dead, to allow more circulation among opponents during deathmatch games by adding openings into the central courtyard.
For the release, the programmers involved made some small tweaks to the source code as required for the new episode, adding the necessary text strings, a fourth-episode demo to the demo sequence, and modifying the boss death triggers to accommodate for new boss situations in the the sixth and final levels. This last change had the side effect of making the modified executable incompatible with a few PWADs that depended on the old trigger behavior. Additionally, the programmers also fixed a glitch existing in previous releases where lost souls would not bounce on the floor or ceiling as intended. This change caused some demos previously recorded on levels including these monsters to desynch.
In other respects the engine performs like Doom version 1.9, and in fact is still marked and internally considered as "v1.9" regardless of the changes mentioned previously, which produce occasional incompatibilities. As such it can run the 1.9 versions of Doom and Doom II, although when doing so the additional fourth demo is still requested by the executable, resulting in the termination of the program immediately following the third demo of the looping demo sequence.
Before the expansion's release and before the name The Ultimate Doom was picked, the game was referred to as The Definitive Doom Special Edition. References to the original name can be found in the source code and in the DOS installer of the game, which features the shortened name DOOM: Special Edition on the titlebar and suggests DOOM_SE as the default installation directory.
In the 2015 Doom source data release, an early version of the Ultimate Doom executable is included in the DoomMapsSE folder of the doom-maps.zip file. When run, its startup displays the title "DOOM System Startup 1.9 Special Edition" in black characters on a cyan background, instead of "The Ultimate DOOM Startup v1.9" in dark gray on light gray background.
- Doomworld.com (1998), 5 years of Doom, interview with John Romero (pg. 3). Retrieved on April 5, 2008.
- Planet Romero (2002), Ultimate Doom thread, id Games Discussion forum, The Romero. Retrieved on April 5, 2008.
- Doomworld.com (1999), Interview with John Carmack, (questions about Ultimate Doom). Retrieved on April 2, 2008.
- Planet Romero (2002), Changes in levels thread, id Games Discussion forum, The Romero. Retrieved on April 5, 2008.