Welcome to Mob Rule. This is a small-to-medium sized pack that has several different objectives.

Objective 1: Bring back the feeling of playing Minecraft for the first time. What's around that corner? What is that sound? What could be out there? After playing Minecraft for even a little bit everything becomes predictable. I remember hearing monster sounds for the first time and expecting a horrible creature to catch me by surprise... that never occurred because Minecraft mobs are generally stupid. When night comes around, you can simply dig a hole in the ground or shut your front door to be safe. Mob Rule changes all of that.

Over 100 new mobs -- all have unique properties. Some are common, others are exceptionally rare.
Completely reworked mob AI. Mobs are scary clever.
Countless surprises that I won't reveal here. They can and will spice up your game.

Objective 2: Make Minecraft more difficult, not through tedious mechanics like thirst or disease, but through thrilling mechanics like intelligent enemies and base building. It seems many of the hardcore packs these days add mechanics like a "thirst bar" or a "sanity meter" that you have to upkeep if you wish to survive. Mob Rule does it differently -- monsters will outwit and outflank you at every turn. When you least expect it, a monster you've never seen before will find a way to reach you. You will have to learn from the behavior of these mobs and develop tactics to overcome them.

Intelligent, scary mobs that tend to always find a way to get to you.
Wildly different tactics depending on which monster you're dealing with (Will it swim underwater to reach you? Will it build a pillar to get you? Will it blow down your door with explosives? Launch itself out of a catapult to get to your height? Work with other monsters to form an army?)
Demands thoughtful base building -- a wooden cube will not protect you from the horrors of the night.
Construct customizable turrets to defend your base, build fences, barricades, and other unique defense features.

Objective 3: Add a meaningful sense of exploration, adventure, and achievement. Minecraft does a fairly good job at implementing exploration, but Mob Rule takes it to a new level. Over every hill, there is a new challenge to be found. Conquer the world... if you can survive.

Dungeons.
Hundreds of overworld structures.
Completely reworked Nether.
Bigger, more interesting cave systems.
Underground biomes.
Countless other surprises that I will not spoil here.

Modlist:

Additional Structures (by XxRexRaptorxX)
AppleCore (by squeek502)
Aqua Creepers (by MrCompost)
Battle Towers (by AtomicStryker)
Better Builder's Wands (by Portablejim)
BetterFps (by Guichaguri)
Bloodmoon (by lumien231)
Chance Cubes (by turkey2349)
Chisel (by tterrag1098)
Construct's Armory (by theillusivec4)
Controlling (by jaredlll08)
Custom Starter Gear (by brandon3055)
Death Quotes (by andrenoel1)
Doomlike Dungeons (by BlackJar72)
Dungeon Mobs Reborn (by Sunconure11)
Epic Siege Mod (by Funwayguy)
Extra Utilities (by RWTema)
Eyes in the Darkness (by gigaherz)
FoamFix (by asiekierka)
Grue (by Shinoow)
Harvester's Night (by Lykrast)
Improved Backpacks (by DreenDex)
Infernal Mobs (by AtomicStryker)
Inventory Tweaks (by Kobata)
Iron Chests (by progwml6)
JourneyMap (by techbrew)
Just Enough Items (by mezz)
Laser Creeper Robot Dino Riders From Space (by Vadis365)
Level Up! Reloaded (by BeetoGuy)
Lootable Bodies (by DrCyano)
Mob Mash (by Akrivus)
Mowzie's Mobs (by BobMowzie)
NoDamI (by ProfHugo)
NetherEx (by LogicTechCorp)
Open Modular Passive Defense (by Keridos)
Open Modular Turrets (by Poenjabiesous)
Primitive Mobs (by Daveyx0)
Quark (by Vazkii)
Roguelike Dungeons (by Greymerk)
So Many Enchantments (by Rin/shelterrin)
Sound Filters (by Tmtravlr)
Spawn Temple (by iastre)
Swing Through Grass (by exidex)
Streams (by delvr)
Tech Reborn (by modmuss50)
The Creeping Nether (by dreadwing93)
Tinkers Construct (by mDiyo)
TorchMaster (by xalcon)
ToroHealth Damage Indicators (by ToroCraft)
Totem Defender (by kopc0801)
ToxicRain (by CoolSquid)
VanillaFix (by Runemoro)

I am always looking for ways to improve Mob Rule. Please leave any suggestions or criticisms in the comments. Thank you for playing!