Rare overpowered mob spawns with random enchants and potion effects that are beyond vanilla limits, at an increasingly lower chance the higher the amplitude of effect or level of enchantment.
You can also blacklist certain enchantments and list thier max level or amplitude when on a rare mob spawn, or enable incompatible enchantments (good for increasing difficulty since you can't tell if its OP or not until you fight it).
They also have a configurable chance to spawn with additional equipment. You add or change available equipment in the config.
