Event scripts files control the game's world state. This is accomplished through events, which are defined by the .emevd files. Events have two broad types of instructions: conditions and results. Event scripts use a custom coding language, which sets up condition groups that wait for certain in-game conditions to be met and then uses result instructions to modify the world state. A simple event might wait until the player is within a certain region and then activate an enemy's AI. More complicated events use event flags to record their progress and interact with other events.
Event scripts should be distinguished from AI scripts, which control how other entities move, attack and respond to the player.
In addition to event scripts, this page also includes information on some debug-related files that are not used in-game (except by the debug .exe) but can be useful for working with event scripts. They are the .emeld files, which provide descriptions for each event in the corresponding .emevd file, and the .emedf file, which documents the commands used as instructions in the .emevd files.
As an aside, there is a file named common.evd which appears to be an event script file like the .emevd files, but is not used in-game. It may be vestigial or could have some unknown purpose. The former seems more likely.
Event script files and their associated documentation files are all stored in event/. For each map, there is a corresponding event script that runs while that map is in the load queue. In addition, each map class (m10, m11, etc.) has a (usually empty) event script file that runs while any of its maps are in the load queue. Finally, there is common.emevd, which runs all the time and usually handles basic game interactions.
Event script files are almost perfectly understood. However, since they rely on a custom interpreter that parses and executes the actual commands, some commands are not well understood or have confusing meanings and results. More comprehensive testing is needed to write documentation for every event script command, and some .exe will probably be needed to determine what certain instructions do.
HotPocketRemix has created a few guide videos on working with DSEventScriptTools and understanding events more generally: Introduction & Basics, Making a Basic Mod (Part 1) and Making a Basic Mod (Part 2).
Additional guides for more advanced topics should be created.