Peaceful Monsters allows hostile mobs like Zombies, Skeletons, Spiders, and more to spawn when your world's difficulty is set to Peaceful. This is achieved by altering some values in Minecraft's vanilla code via Mixins from SpongePowered.
Some examples of this include:
- Redirecting the difficulty checked by spawners to never be Peaceful, so they are always enabled
- Injecting into the melee attack goal used by mobs and monsters to disallow attacking in Peaceful
