Hard difficulty level gives +20% bonuses to the AI. Is it possible to create a custom difficulty level by modifying a file? For example, giving bonuses of 10% instead, thus creating a new difficulty level between Normal and Hard?

This is absolutely doable in some way or another.

At the very least, you could mod the settings for Hard to be whatever you think is appropriate.

You can check the public folder of your game installation directory (to make edits directly to your game files, which I wouldn't reccomend) or you just create a new mod (with the help of the modding help threads here) to overwrite those settings.

Yes, there would probably be less likelihood of any issues if Hard difficulty were to be nerfed, rather than creating a new difficulty setting with its own name.

There's this page on the wiki which details what the difficulty setting modifiers are: http://endlessspace.wikia.com/wiki/Game_settings

I guess that firstly one would want to know if this page lists everything governed by the difficulty level, or whether there's other stuff not listed. Then one would need to know which file contained these settings, and thus far I haven't managed to locate it.

Oh, no, it's definitely in the public files. I don't have Endless Legend installed right at the moment.

For Endless Space 2 it is located at /Public/Simulation/SimulationDescriptors[GameDifficulty].xml

It's probably the same (or somewhere similar) for Endless Legend.

I've been doing some more digging through the XML files. The file GameModifiers[GameDifficulty] lists three paths that appear to have their own settings related to difficulty level. These paths are (for Serious difficulty):

GameDifficultySeriousHumanMajorEmpire
GameDifficultySeriousAIMajorEmpire
PersonalityMajorFactionGameDifficultySerious

These paths, and their parameters and settings, are found in the following files (the first two paths in the first file and the third path in the second file):

SimulationDescriptors[GameDifficultyBonus]
Personalities[GameDifficulty]

So, hopefully, by comparing the various parameters and settings for each path and difficulty level it should be possible to modify the numbers to make a custom difficulty level.

Anyone know whether it's possible to edit core files directly or would a mod have to be created? Some games let one edit core files directly, while others have the public files just for show and the actual files the game uses are buried and locked away elsewhere.

I don't fully understand how to create a new mod, despite reading the tutorial pdf.

I have the Improved AI mod installed, which uses both of the XML files that will need to be modified to create a new difficulty level. So that would be another complicating factor in creating a new mod, regarding which files the game would use.

So I think it would be simplest for me to edit these two files to my liking in the Improved AI mod, thereby piggybacking it for my own use. I would therefore be sure that my changes would appear in the game and that there weren't any conflict issues with the other mod. I'll just have to make sure everything's backed up.

I would check the Amplitude Discord and ask for help there. Or follow a guide and set some time aside to learn and work through it.

It's possible, I added a lot of new difficultys in my "progressive difficulty" mod.

but there is a problem with another file (a quest file), using a test about difficultys.

Problem is the main file is not updated and I can't write the plug-in link to that file.

So it will make main quests of two factions not working with the mod (instead of propose next quest, I presume game will not crash but you will never get the next step then never be able to complete the main quest).

Amplitude have to release the updated main file for making that work well.

I could remove new difficultys and tweak difficultys differently for solving the case. Maybe I will do that when updating my mod, but Im not working on that for now.

As players don't play below serious difficulty, I will just rank that with "easy" become "serious" and so on.