Section 1: What is a level mod?

In order to untangle the mess that is teaching the pre-requisite knowledge of being able to create level mods on your own, it is best to lay down the process step by step.

This is something I've noticed this wiki does not do. It may have a lot of useful documentation such as eventcalls and miscellaneous entities such as damage triggers, particle effects, etc., but it does not put together a concise tutorial on how to use them properly.

This guide sets out to correct this.

First, a brief write-up of what exactly level mods are and how they work.

TL;DR: A level mod replaces the "level file" within a map's .resources file(s), which contains the sum total of all of its assets.

A single map in Doom Eternal has the sum total of its assets and files located within its RESOURCES files, with the file extension ".resources".

A RESOURCES file is what you would imagine it would be, a file that is jam packed with a huge amount of assets and resources that, when combined and used by the map, form the entire experience of playing through it. 

You can even see them for yourself. On the Steam/Windows version of Doom Eternal for example, the RESOURCES files for the World Spear for example are located in:

C:\Program Files (x86)\Steam\steamapps\common\DOOMEternal\base\game\dlc2\e5m1_spear

The _patch files are merely extensions of the main file and are noticeably smaller in size, although _patch2 is larger because it contains all of the World Spear's official Master Level assets.

These RESOURCES files, when combined, have everything the map needs to function properly. They have everything from particle effects to models to, most importantly, the file that determines how the map operates when loaded.

This file, with the file extension .entities, is what id Software themselves change when they push a new master level out.

This file is highly important, it takes all the individual assets in a map's RESOURCES file and mashes them together into a single playable experience. Particle effects, hazards, enemy encounters, triggers. Everything.

So all in all, these RESOURCES files, especially the .entities file, are pretty important then. The only thing they don't have is the map geometry itself.

Level mods edit these RESOURCES files directly. Specifically, they replace the .entities files within. You know, the thing id Software themselves change when they push a new master level out.

Next step, we will gather the required tools needed to create level mods in a fast and efficient way. Don't worry, it's not a lot!