Each game 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 -skill parameter.
This is the name or tag for each skill level according to the source code.
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 behavior 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.
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, an energy cell will provide 40 units, and a box of bullets will provide 100 bullets instead of the usual 50.
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, and that monsters do not respawn on the "Black plague possesses thee" skill level like they do in Doom's "Nightmare!".
The Marine takes 75% of the base damage. Slightly fewer monsters are encountered.
The Marine takes 90% of the base damage. Normal number of monsters.
The Marine takes 170% of the base damage. Health kits are fewer in number. More monsters appear in this skill level.
The Marine takes 350% of the base damage. The player starts with the Soul Cube in Doom 3, or all Artifact powers in RoE. All medikits are removed from the game, and the player's health steadily drains until it's 25 or below. The only sources of healing are health stations, or the healing powers of the Soul Cube (Artifact in RoE). Number of monsters is the same as on Veteran skill. This skill level must be unlocked by first completing the game on any other skill level.
In addition to the five Doom standard skill levels, in vanilla Doom a skill level of 0 can also be specified, but only using the -skill parameter. Doing this empties maps of all things apart from the player starts. This does not work in many source ports. It also renders many levels that have boss trigger effects, require keys or use teleporters impossible to finish without the aid of cheat codes.
Early versions of Doom did not support "Nightmare!" skill. The initial mail-order releases were version 1.1 and only included the first four skill levels. The widely circulated version 1.2 added "Nightmare!" skill support. "Nightmare!" was added with the characteristic dark humor of the Doom games in response to any comments saying the game was too easy on "Ultra-Violence." After selecting "Nightmare!" from the main menu, the player is asked "Are you sure? This skill level isn't even remotely fair." The player must then confirm that he wants to play on Nightmare mode, a step which is unique to this skill level.