Year Without a Summer (en.wikipedia.org)

Stumbled across this while reading about Caspar David Friedrichs change in the color composition. The event also lead to Mary Shelley to write "Frankenstein"

For those unfamiliar with history - phrases such as "profound famine" can be translated to "tens of millions of people died, slowly, and in horrific circumstances". Though I wouldn't be surprised if the total death toll, 1816-1820, exceeded 1e8.

For reference, the total world population at the time was only ~1e9.

Note also Wikipedia's long list of links to similar events.