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:

SkillMenu nameSource codeThing placementExtras
1I'm too young to diesk_babyIdentical to Hey, not too roughHalf normal damage, ammo is doubled
2Hey, not too roughsk_easyLeast numbers, strength, or complexity
3Hurt me plentysk_mediumDefault numbers, strength, or complexityDefault chosen
4Ultra-Violencesk_hardGreatest numbers, strength, or complexity
5Nightmare!sk_nightmareIdentical to Ultra-ViolenceAmmo 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)...