This is a spiritual successor to the cursed earth that was included in the Extra Utilities 2 mod.
Cursed earth is created by right-clicking dirt with a drop of evil, which is a rare drop from wither skeletons. Once created it will die out if in sunlight, but in darkness (or torch light) it will spread to surrounding dirt blocks.
In darkness (or any light level of 7 or below) it will rapidly spawn the hostile mobs that would naturally spawn in the same biome as which the cursed earth is located. The same spawn weights will be used (so in plains spiders, zombies, and skeletons are common with witches and endermen being less common). Any hostile (monster) entities added to a biome's natural spawn tables by other mods should be spawned as well.
The Fabric loader and API are required.