This feature doesn't work on servers that run Java 12 or later.
Spawn Conditions are a way for you to override the vanilla conditions system, and configure conditions of your own.
To add a mob to override, simply create a list for a mob under the spawn-conditions option in the entities section of the config.yml. You can find a list of entities that correspond to the version of your server by clicking here.
# Setting an empty list to a mob will remove all spawn conditions
# This example will remove all other spawn conditions and only have NOT_PEACEFUL as a spawn condition for Zombie.
This is a list of spawn-conditions available by default.
Note that not all these conditions exist, depending on your server version. Be sure to check Mob Group!
Checks for Light Level above 8
Checks for Light Level above 8, but also checks for a Cold Biome
Checks for Below Sea Level
Checks for Light Level below 8
Checks for Lava Block and Air above Spawn Location
Checks if the Y-Level is the Sea Surface
Checks for Slime Chunk or Swamp Biome
Checks if Location is in Deep Water
Checks for Monster Light Restrictions, this has more conditions than DARK_BLOCK_LIGHT
Checks for Biomes that aren't Ocean
Checks for Y-Level above 45, for not too Deep Oceans
Checks for Blocks that are not on a Nether Wart Block