“There was one who came to rest on a bleak moor, strewn with the dead and scattered with the carnage of battle for leagues in every direction. This was the world of Barbarus. A planet perpetually shrouded in poisonous fog, whose mountainous crags were ruled by warlords with fantastic powers and horrific appetites, and whose human settlers, stranded there millennia before, were crowded into the lowest valleys, beneath the choking mists. They lived lives of unrelenting terror, eking out a peasant’s existence by day beneath a dim sun which never burned completely through the fog, and cowering by firelight after dark from the terrible beings which moved unseen above.”
Find more at Warhammer 40K Wikia and the Lexicanum.