Ultimate Super Doom 3 (Also known as USD3) is a mod for the original DOOM games by Doomero. The mod's primary purpose is to port weapons and enemies from Doom 3, including weapons and enemies which were present in Doom 3 but not in the original games. Unlike some other mods, Ultimate Super Doom 3 is much more complex, importing not only the weapons and enemies but also much of their behaviour from Doom 3: for instance, the BFG 9000 can be charged to release a more powerful shot.
To make multiplayer more interesting, Doomero has disabled Monster infighting, although in certain cases this can alter gamplay (such as the room in MAP08: Tricks and Traps in Doom 2 containing the Cyberdemon and the Barons, which require triggering monster infighting to succeed).
The PDA is included but its functionality is changed: instead of containing crucial information from several key characters, it instead contains a list of credits. In one of the additional mods the PDA also serves as a satellite video uplink showing certain videos from Doom 3 (such as Sarge's messages to the marine).
Included with the mod is the ability to play Ultimate Doom straight through instead of in episodes.
- All enemies are replaced with their Doom 3 equivalents. Enemies will largely exhibit their Doom 3 behaviours.
- Monster infighting is disabled.
- Some enemies are decided at random: for instance, where an Imp may usually spawn, instead a Vulgar might spawn in its place, or instead of a Hell Knight, a Bruiser may spawn instead.
- Certain enemies not present in Doom 3, such as the Spiderdemon and the Arachnotron, are upgraded.
- Certain enemies are replaced entirely: the Baron of Hell is replaced by the Hell Knight, and the Lost Soul is replaced by the Forgotten One.
- Pain Elementals do not explode upon death, and they do not release a horde of Forgotten Ones.
- The Berserk pickup changes the perspective of the player briefly, as in Doom 3. With this powerup, you cannot change weapon until it wears off. All attacks on enemies while the Berserk powerup is active will kill them in one hit.
- Several types of Z-Sec are included: the only one excluded are the Riot Shield Troopers. None of the civilian zombies are included.
- An additional mod adds certain cutscenes into Ultimate Doom. This mod can be played in conjunction with, or aside from, the mod allowing the player to play Ultimate Doom as a whole game.
- The Soul Cube or the Artifact is available to the player on certain difficulty modes.
- The Cyberdemon, while invulnerable in Doom 3, is made vulnerable.
- The Super Shotgun is made available in Doom.
- The flashlight is included and as in Doom 3 can be used as a weapon.
- Dead bodies can be found throughout each level - these can be used to power up the Artifact.
- The next version of Ultimate Super Doom 3 to be released by Doomero will be called Revenge of Evil. It is to be a reimagining of Doom 3's storyline with new maps and monsters.