Find the indicated floppy disc collectible in the listed mission to unlock the corresponding cheat code. Note: Cheat codes can only be activated after completing a mission. Cheats can be activated at the "Mission Select" menu at the tower. Cheats do not disable progression. However, Slayer Gates are disabled when using cheat codes.
All Runes: In the "Fortress Of Doom", use batteries to unlock the right tower, then drop down to a hidden ledge.
Famine Mode: In Mission 10: Nekravol, in the area where the Marauder spawns, drop down into a hidden room where the cages of humans are being moved up. This cheat causes enemies to not drop armor or health pick-ups.
Fully Upgraded Suit: In the "Fortress Of Doom", in the Power Room near the Control Room, shoot the red switch in the window to reveal a jump pad. This cheat unlocks all Praetor Perks.
IDDQD: In Mission 3: Cultist Base, in the chamber with the eight cages you have to melee open, use the yellow poles to reach the upper platform in the central chamber. This cheat unlocks Sentinel Armor.
IDKFA: In Mission 6: Arc Complex, after leaving the main Convention Center, you will need to swing on a yellow pole. Just ahead will be some enemies coming out of a hole. Look in the enemy spawn hole to find the cheat. This cheat unlocks all weapons and equipment.
Infinite Ammo: In Mission 5: Super Gore Nest, at the entrance to the huge Gore Nest with three keys, look on the cliff to the left when facing it. There is a small hole with a cracked wall you can punch through that leads to this cheat.
Infinite Extra Lives: In Mission 1: Hell On Earth, in the subway maintenance tunnels, look right when you reach a t-junction with a tentacle to your left to find a crate. Jump up and smash through the vent to find the cheat.
Instant Stagger Mode: In Mission 11: Nekravol Part II, Just below the top of the Argent Tower, after the Crucible charge, there is a small landing with the cheat and a Praetor Point. This cheat makes weapon damage and dashing instantly staggers all standard enemies.
Party Mode: In Mission 11: Nekravol Part II, past the Doom Hunter arena, go through a hall to a deep shaft. Right below the entrance is a cracked wall. Drop down, double-jump, melee the wall, and dash to grab the ledge. Otherwise, you will need to reach the end of the mission and fast-travel back. This cheat causes confetti to shoot out of severed monsters instead of blood.
Power-Up Mode: Berzerk: In Mission 12: Urdak, at the purple array station, look for a cracked wall to your left before using the exit launcher. This cheat gives infinite Berserker power-up, but it is only available in certain missions.
Power-Up Mode: Onslaught: In Mission 7: Mars Core, after launching yourself out of the catapult and crashing through the UAC facility wall, take the lower path to a jump pad. You will launch up through a ruined floor with the cheat. This cheat gives infinite Onslaught power-up.
Power-Up Mode: Overdrive: In Mission 9: Taras Nabad, in the Archvile catacombs, take the ramp out of the arena and smash through the cracked wall. This cheat gives infinite Overdrive power-up.
Quakecon Mode: Unlock all suit upgrades with Praetor Points. This cheat causes an audience to cheer whenever you score kills/glory kills.
Silver Bullet Mode: In Mission 4: Doom Hunter Base, climb to the top of the tall cylinder that contains the sarcophagus with the Doom Hunter to find the cheat. This cheat makes weapon damage and dashing instantly kills staggered standard enemies.
Successfully complete the DOOM Eternal campaign to unlock the fully playable original DOOM on the computer in Doomguy's room in the Fortress Of Doom. Note: The cheat codes unlocked in DOOM Eternal can also be used in DOOM.
Classic DOOM 2
Enter the Fortress Of Doom hub, and travel down to find Doomguy's room with a special computer that you can access two mysterious "?" files on it. Those two files are the fully playable, complete original versions of DOOM and DOOM 2. Enter "FLYNNTAGGART" as a code to unlock DOOM 2. Note: The cheat codes unlocked in DOOM Eternal can also be used in DOOM 2.
Fortress Of Doom
Successfully complete Mission 2 to unlock the Fortress Of Doom hub area, which contains many secrets and Easter Eggs.
Secret Unmaykr weapon
Collect all six Empyrean Keys from the six Slayer Gates in Missions 2, 3, 5, 6, 7, 9 to unlock the secret Unmaykr weapon in the Fortress Of Doom. The Slayer Gates are circled in purple on the map of these missions. To open the Slayer Gates, you must find their nearby Slayer Key in the respective mission. Empyrean Keys drop from Slayer Gates. Slayer Gates are very difficult combat encounters. If you are having trouble completing Slayer Gates, you can replay any mission from the "Mission Select" menu. Just return when you are more powerful. The Unmaykr gun case can be found by going to the center chamber in the Fortress Of Doom and dropping down to the lowest level, then go forward to the tall chamber. The Unmaykr uses the same type of special ammo as the BFG-9000. However, against some enemies it is much better since the ammo lasts longer than the BFG. It sprays laser bullets in a wide angle in front of you and deals massive damage, more than any normal weapon. It is a great weapon for later in the game. The Unmaykr first appeared in DOOM 64 as a secret weapon. You could find the Unmaykr (or Laser Gun) in secret levels, and upgrade it into an amazing gun by collecting hidden keys.
Easy Mastery Challenges
After unlocking a Mastery Challenge, you can easily farm the challenge using the following steps:
1. Start a mission from "Mission Select" at the main menu or on the Doom Tower. Go to a mission that has an easily accessible battle with the enemy type you need. For example, Mission 1: Hell On Earth features an Arachnotron in the first city Demonic Infestation arena. For the Sticky Bomb Mastery, you need to break the weakpoint with Sticky Bombs from the Shotgun 25 times.
2. Go to the arena with the weapon and enemy type you need to farm. The more of the enemy type, the easier farming is. However, you can farm even if only one enemy type spawns.
3. Complete the Mastery Challenge —- make sure the Mastery Challenge meter goes up by 1 or more. Look for the notification.
4. Pause the game and select "Load Checkpoint" to restart the arena. You will refresh all ammo you had before, but you will retain the Mastery Challenge completion.
5. Repeat the process until you complete the challenge.