The Hardest Pack

This modpack is the hardest one on the market. Not only do the mobs have massive amounts of health, but they also deal insane amounts of damage. There are also many other changes to increase the frustration of your playthrough. For example, zombies can smell you, carry creepers on their heads, and carry firearms. You will never be safe. Even breaking grass can spawn deadly fiends. 

Features:

Overhauled Health system: This pack stars First aid and rough tweaks. Your health is split into five parts: your head, torso, arms, legs, and feet. If your head or torso runs out of hearts, you die. All of the others produce debuffs when they run out of health. Rough tweaks disables natural healing. You now need items to heal yourself.

More insane difficulties: Thanks to the mod, More Difficulty options, you can make the game even more hellish.

Several dungeons and dimensions.

Temperature and thirst

Tons of new enemies to fight.

And no Ancient Warfare, sadly.