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.

Design
The expansion's design was lead by John Romero with American McGee and Shawn Green, and he recruited two prominent level designers from the fan community to complete the team, John "Dr. Sleep" Anderson (who would later help John Romero with Daikatana) and Tim Willits (who eventually became id Software's lead designer), while the artists did a quick job on the few extra graphics required for completion.

Since the expansion was produced while id Software was involved in other projects, not much thought was put into making it consistent with the previously developed plot of the game (which included Doom II, a portion that occurs after the added episode), and the designers simply concentrated on straightforward action (which was already the general focus of the games , in any case). As a result the locales of the levels are unclear, and one is not certain if the Doomguy is still in Hell, already on Earth, or somewhere in between. The final message and screen with the rabbit Daisy arguably reinforces the acknowledgment that the designers' main concern at that point was the levels and not the background story or any general aesthetic considerations.

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. Additionally, since Doom II features were present in the executable, some of these (namely switches, linedef actions, and fast doors) were used in the new levels.

Engine
Michael Abrash joined Romero in order to make some tweaks to the source code 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 versions 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.