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Really hardcore start. You will not be able to craft your Crafting Table or Chest or Barrel or tools right away.
Blocks can be broken with appropriate tools only.
Almost every important vanilla recipe is altered.
Sleep mechanic is changed
Hordes of smart zombies roam around at night. And you cannot skip the night that easy as before!
Every item left in world will turn into Silverfish instead of just de-spawning.
Water is a valuable resource and is finite if you're not in a "Watery" biome.
Don't left your farmland unoccupied, Poison Hemlock is waiting for one to grow on.
Animals will fight back if you treat them without respect.
Water is even more dangerous for you to be.
Building underground isn't a good choice. Zombies will break your walls.
Rich amount of weaponry for you to chose from (not only swords and bows, but also Refiles! Yay!)
About 100 quests in a quest book (only in Russian for now). More like a hint on how to progress in a modpack.

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You'll have to visit Blue Skies dimensions and the Nether and the End of course, just for obtaining much needed Ores.

Space travels courtesy of Ad Astra is a must-have.

You'll not be able to progress in one mod without other. All recipes are criss-crossed.

Create, Immersive Engineering, Thermal Series, PneumaticCraft need to be completed to reach the ability to craft the Final Goal Item.

Computer Craft, Refined Storage, Integrated Dynamics and Hostile Neural Networks are there to help you.

Sophisticated backpacks, Animalistic and Immersive Airships will help in your adventures.

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Weird Mobs, Dungeon Denizens, Creeper Overhaul, Zombie Mobs and much more monsters are there.
You're Seeing Dungeons for you to develop your combat skills even more.
Farmer's Delight and Infernal Expansion will add to your ration and Spice of Life: Carrot edition will aid in getting additional hearts.
Are you a Trovo streamer? Well, Trovogration is there, fully configured to support all tricks and treats of the modpack.
A whole bunch of Minecraft optimization mods included, so your PC won't blow up.

The Infestation