[In] clear and calm weather, a black haze of sand appeared to the north of the mountains. The cloud was so extensive that in a short time it had spread over the entire area and so thick that it caused darkness indoors. That night, strong earthquakes and tremors occurred. 
The earth had ripped apart along a sixteen-mile fissure later called the Laki volcano. This fissure intermittently continued to spew lava over the next eight months until its last eruption on 7th February 1784. The results of this eruption were felt across Europe, parts of North America and reportedly as far afield as Asia and North Africa. Understandably, the effects were worst in Iceland where an estimated quarter of the population died as a result.
Diaries and letters of the time help the historian trace the impact of the volcano. In Norway, on the 10th June, Johan Brun (also a Lutheran priest) wrote in his diary about black ash that had fallen from the sky causing plants to whither. Over the next week the dust cloud reached Prague and Berlin. An apparent change in direction of the prevailing winds, meant the ‘dry fog’ then passed over France on the 18th before it arrived over Britain on the 22nd.
Wherever the cloud appeared, those people who worked the land became ill. As the English poet William Cowper wrote in a letter to Rev. William Unwin in September 1783:
[S]uch multitudes are indisposed by fevers in this country, that the farmers have with difficulty gathered in their harvest, the labourers having been almost every day carried out of the field incapable of work; and many die. 
The climactic impact of the eruptions were felt for many years, and may have contributed to the poor harvests across Europe in 1788, which are often seen as a key cause of the French Revolution.
To learn more about the Laki eruptions and their environmental impact, see this excellent article on The Economist web-site.
 J. Steingrimsson, K. Kunz (trans.), Fires of the Earth: The Laki Eruption 1783-1784 (Reykjavik,