A modpack based around the idea of "how can the player's experience be made more nightmarish in as many ways as possible, while still retaining features apart from mere difficulty?"

First, the darkness. Hardcore Darkness removes the minimum light level, while Advanced Darkness plunges the world into perpetual gloom; and, as if that was insufficient, Grue will spawn in the dark! Oh, and your torches burn out. Isn't that lovely?

Next, we have hunger. Hunger Overhaul makes the acquisition of food into an actually challenging task, while Spice of Life (no, not the carrot addition) makes your stockpiled food essentially worthless if gathered in insufficient variety. Add in whole seasons in which most foods can't be grown at all... well, you'll be extra glad that Pam's Harvestcraft is around.

Just for fun, you also need to manage thirst and temperature (courtesy of Tough as Nails). While some mods have been added to make this a bit more manageable, the aforementioned seasons certainly are not among them.

There's also Weather Storms and Tornadoes!

Most mobs have been buffed in some way (Rough Mobs 2, Zombie Awareness), and numerous mods expand the selection even further (Ice and Fire, others). Additionally, mobs will be buffed according to numerous factors. There are also Weeping Angels.

Health has been significantly reduced, down to three hearts to start with. Also, natural regeneration is disabled -- however, some methods have been added to replenish health.

Skill requirements have been added (Reskillable) for many things, so XP is important. There are also a few extra uses for XP, which I'm sure you can find.

Crafting recipes, particularly early-game Vanilla ones, have been significantly altered.

And that's it, as far as difficultly enhancement goes!

There are several technology mods, Endgame Plus for adventuring, Apotheosis so that you might someday be more powerful than your foes, and numerous mods to improve one's quality of life and the game's atmosphere. We want the difficulty to be from the gameplay, after all, not the system!

Have fun, and be sure to persevere!

A Cold World