Create: Radiation n' Zombies
A world infected by radiation and the undead, where only You and the Power of Create remains.
Intelligent mobs
Zombies, skeletons and creepers are much stronger now: They are smarter, don't burn in sunlight, and track you down more easily. They are able to breach your walls, and are infectious.
Protection suits
Because of the constant radiation, you need to wear protective suits. You can also craft filter blocks to lower the radiation in your base.
Weapons
You can craft guns to protect yourself from mobs. It is not enough against a horde though...
Giants
Giants now spawn naturally, and they are strong.
Cannons
Cannons are a great way to defend your base against breaching mobs.
Radiation
Every biome has a radiation level that determines its lethality. Lush caves are completely safe, and you may survive a few minutes in a forest, but without proper equipment, you die almost instantly in a desert. Also, avoid Basalt Deltas at all costs!