The Dimmendark was an ever-expanding zone of "winternight" that was generated by the Myrkenstone during the Winter War. Modru created this effect to allow his Foul Folk to roam freely across the land, without having to worry about the Ban killing them when the sun was up.
It had the additional benefit of being very difficult for most other species to see through. Only Warrows were able to see for a significant distance through the Dimmendark, due to their special secondary eyesight. Humans were able to see very little, and Elves somewhat more.
Despite its usefulness, this tactic was only utilized during the Winter War.