Skill level

Each game's skill level provides general difficulty settings that allow a greater variety of players to enjoy the game, allowing novices to face fewer opponents even with advantages for the player, and giving those who have mastered the game a challenge against many monsters, possibly with enhanced aggressiveness. Each skill level bears a colorful name suiting the character of the particular game being played and can be selected when the game starts or through additional options, particularly the parameter.

Doom and Doom II skill levels

 * 1) This is the name or tag for each skill level according to the source code.
 * 2) This is the name of the graphics lump in the skill menu.
 * 3) Each thing placed in a level (except for player starts) has cumulative bit flags (easy, normal, hard) marking on what skill levels it appears. The difference mainly amounts to larger and tougher comportments of monsters on the harder skill levels, but may also account for changes in the location of certain things. For example, in E3M6: Mt. Erebus the blue key is located in different places depending on the skill level.
 * 4) Double means twice as much ammo is added to the player's current total when a corresponding item is picked up. For example, a rocket will provide 2 rockets, 4 shotgun shells will be counted as 8 shells, and a box of bullets will provide 100 bullets instead of the usual 50.
 * 5) All cheat codes except and  are disabled. Some source ports may change this or have internal workarounds.
 * 6) Monsters attack more rapidly, demons and spectres move faster and several monster fireball types fly faster.

The 2019 re-release introduced a new skill between Ultra-Violence and Nightmare!, entitled Ultra-Violence+. It behaves as UV with fast monsters but also enables the spawning of multiplayer-only map things in single player mode.

Doom 64 skill levels
The "Nightmare!" skill level, introduced in the 2020 re-release of the game, is not directly accessible from the main menu, being only available by having "skill 4" as a launch parameter, alongside the -warp command. It works correctly with savegames, and the name of the skill level will be displayed upon accessing the save/load menus. It does not, however, work correctly with the password feature.
 * 1) Called "Hardcore!" in Doom64 EX.
 * 2) Unlike the "Nightmare!" skill level in the first two Doom games, there is no block against cheats and no monster respawn feature. Combining skill 5 with the -fast parameter adds a special check in the monster behavior code causing all monsters to attack immediately on sight, similar to how monster behavior works in the previous games.

Heretic skill levels
See the Doom and Doom II table above for specific notes, as there are no differences in general, except that +50% means one and a half times as much ammo is added to the player's current total when a corresponding item is picked up.

Hexen skill levels
The alternate righthand titles for the Cleric are used on the N64 port, due to Nintendo Of America's strict rules on religious imagery in games.

See the Doom and Doom II table above for specific notes, as there are no differences in general.

Strife skill levels
For other relevant notes, see the Doom and Doom II table above.
 * 1) Acolytes and stalkers move, attack, and react to pain half as fast. Reavers and ceiling turrets attack half as fast. Crusaders attack and react to pain half as fast. The Bishop's homing missiles move twice normal speed (see Bishop's missiles have reversed skill properties).
 * 2) All monsters attack aggressively. Acolytes move, attack, and react to pain twice as fast. The Bishop's homing missiles move at half-speed.

Hacx skill levels
All skill characteristics are the same as for Doom.

Chex Quest series skill levels
All skill characteristics are the same as for Doom.

Doom 3 skill levels
The difficulty setting can be controlled by the console variable. The damage changes take effect immediately, but a map restart or change is necessary for the rest. For example, if a player begins the level on Recruit difficulty and then enters  in the console, immediately their health will begin its drop to 25 and they will receive Nightmare damage. However, the Soul Cube is not given, med kits remain in the level, and the amount of monsters does not increase.

Nightmare mode must be unlocked by completing the game first. This can be bypassed by changing the console variable.

Doom Eternal skill levels
Doom Eternal shares its skill levels with its predecessor, with the addition of Extra Life Mode. Similar to Ultra-Nightmare, Extra Life Mode is a Permadeath mode that can be applied to any skill level save for Ultra-Nightmare. When it is active, running out of extra lives will cause a Game Over in the same manner as dying on Ultra-Nightmare, erasing all progress in the campaign.