In Doom, upon starting a game, you are prompted for the skill level, which is how hard monsters are placed, how much damage you take, etc.
Here is the data:
|Skill||Menu name||Source code||Thing placement||Extras|
|1||I'm too young to die||sk_baby||Identical to Hey, not too rough||Half normal damage, ammo is doubled|
|2||Hey, not too rough||sk_easy||Least numbers, strength, or complexity|
|3||Hurt me plenty||sk_medium||Default numbers, strength, or complexity||Default chosen|
|4||Ultra-Violence||sk_hard||Greatest numbers, strength, or complexity|
|5||Nightmare!||sk_nightmare||Identical to Ultra-Violence||Ammo is doubled, all cheat codes (excluding idclev and iddt) are disabled, monsters attack more rapidly, monsters respawn using their dead corpses|
Nightmare! will really give you nightmares, because monsters actually respawn despite you may think that they teleported, monsters can kill you instantly, all cheat codes are blocked (for levels like E4M3 or E4M7, 100% secrets is impossible without turning on noclip, which makes a NM100S run invalid), but at least ammo is double and you can get over 100% kills due to monster respawning, however the skill level was NOT included with the old versions of Doom, because it was only 4 skill levels until someone contacted id Software saying "Ultra-Violence is extremely easy!" or something like that yeah, yeah, so an impossible skill was added in (I don't know which version of Doom was that)...