I decided to create a few Mutants heavily inspired by the Mutant Creatures/Beasts mod. These only spawn on Hard difficulty with a very small chance to replace the usual mobs. They are very challenging and they should be fought with good gear. More mutants may be added in the future.
Requires Geckolib
Mutant Spider: The fastest of the mutants, they dash quickly and shoot webs to trap players. They drop a Ball of Webs that can be used to shoot webs at enemies. On death they drop an egg that explodes into smaller spiders.
Mutant Cave Spider: Has a rare chance to spawn from regular spiders, they have a powerful venom spit attack as well as a quicker dash. One of their melee attacks is replaced with a bite. Drop is the same as the regular Mutant Spider.
Mutant Witch: Can cast spells from her Cauldron Scepter to put a random negative effect on players. She can also summon a bunch of fiery bats to home in on opponents. Her healing is pretty strong so you have to be aggressive. Drops the Witch's Nose, which can be brewed with an Awkward Potion into Mutant Brew which gives a bunch of positive effects.
Mutant Allay: They will attack all hostile mobs with slash waves from their blades. These waves can be horizontal or vertical, vertical explode and set on fire on impact while horizontal are more accurate and deal less damage. They follow players holding cookies and drop nothing on death.
