ChernobylCraft.

This modpack is designed to be tough, with much more difficult mobs, constant radioactive fallout, spoiling food, and massive ruined cities. Don't get too close to the Рудий ліс, they say it takes 5 minutes before the radiation makes you drop dead. Starting with only a pistol and a gas mask, make your way to the extraction point (not yet implemented) before you succumb to the radiation.