The largest death toll due to a lightning strike happened on April 3, 1856 when a bolt hit the Palace of the Grand Masters on the Greek Island of Rhodes. The building was being used to supply ammunition for the Ottoman Empire and several people sought shelter there during a massive storm. About 4000 people in or around the structure were killed when the lightning set off the ammunition in a devastating explosion.
The second deadliest lightning strike happened years earlier under similar circumstances. The Church of San Nazaro in Brecia, Italy was being used to store over 200,000 pounds of gun powder for the army of Emperor Napoleon. A lightning strike in August 1769 ignited the gunpowder and the resulting blast and fire destroyed the town and killed about 3000.