Dawnless Day

The Dawnless Day was an event at the end of the Third Age. It can be seen ingame in the regions of Dead Marshes, Central Gondor, Eastern Gondor, Old Anórien and Far Anórien.

With the release of March of the King, Old Anórien can be experienced without the Dawnless Day.
With the release of King's Gondor, the areas of Lossarnach, Upper Lebennin, Lower Lebennin, Dor-en-Ernil, and Ringló Vale can also be experienced without the Dawnless Day.
The region of Far Anórien and the area of South Ithilien (Eastern Gondor) are currently only accessible with the dark skies of the Dawnless Day.

Lore

Sauron sent dark clouds from Mordor that veiled the sun in an attempt to crush morale of the Gondorians before the Battle of the Pelennor Fields. The Dawnless Day lasted for five days, and ended on the day of the Battle of Pelennor Fields.