Difficulty of Nightmare

Reinforces the enemy to a level that is impossible to clear.
Enemies are enhanced in attack damage, movement speed, maximum strength, search range, swim speed (swim mobs only), protection, knockback, flight speed (flight mobs only), knockback resistance, and automatic jump height.
These can be changed in the config to change the multiplier. (Default is 10x)
Some mobs also become children or become more explosive.
And almost all attackable mobs are hostile to the player!