Deep within Minecraft's code, where people seldom venture, lies a relatively-unknown system called 'regional difficulty'. Thanks to its existence, the difficulty in different areas is able to steadily progress - up to a point, of course - in various ways depending on things such as the world's selected difficulty, how much time players have cumulatively spent in the area, and even the phase of the moon. Near the start of a new world, one would likely not remember encountering zombies and skeletons wearing enchanted armor, or creepers and spiders that have spawned in already affected by potions effects, yet players that spent ages perfecting large-scale builds would have certainly noticed such occurrences popping up more and more over time.

Those examples are notable symptoms of regional difficulty… and Restless Legions inverts it.

With Restless Legions installed, the regional difficulty of all chunks in the world is no longer 0 by default. Neither does the regional difficulty of specific chunks increase with player presence there - just the opposite. Restless Legions makes it so the world is a hostile place from the very start, with the aforementioned symptoms becoming encounterable immediately, and areas can only be made safer if players have spent a substantial amount of time in the vicinity.

Compatibility: This mod uses mixins to overwrite the vanilla calculateDifficulty method of DifficultyInstance.java. Therefore, any mods that change it too are practically guaranteed to cause a compatibility error. However, the method in question is so niche that practically no one ever touches it - and, if they do, they merely call it instead of modifying it - so it should be expected that essentially no mods in existence would have any issues with Restless Legions.

Testing Disclaimer: It should be pointed out that skipping time via commands does not affect the habitation value of chunks. When you see the 'local difficulty' field in the F3 menu change by using time-skipping commands, that is only happening due to the moon phase factor - different phases of the moon change the regional difficulty of chunks differently.

Instead, if one wants to test how the habitation factor affects the value, unfortunately the best method that I am aware of is by manually staying in the same chunk for a long time after raising the tick speed via the appropriate command to as high as possible.

The gradual increase of the habitation value is also player-dependent, so it's easier to test it in multiplayer: for example, two players staying in the same chunk increases that chunk's habitation value twice as fast compared to if there was only one player.