While many sites offer "free downloads," Cities: Skylines is a paid game that occasionally goes on sale or appears in bundles. To play the Natural Disasters DLC, you must first own the base game. You can find official downloads and deals on:
Surviving the Storm: A Guide to the Cities: Skylines - Natural Disasters DLC cities-skylines-natural-disasters-pc-game-free-download
: Brace for tornadoes, earthquakes, sinkholes, and forest fires. While many sites offer "free downloads," Cities: Skylines