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The Dark Years was a term for the long years of hardship of the Men of Middle-earth during the Second Age. While Númenor flourished, the Men of Middle-earth endured trouble and war, and many of them turned to the worship of Sauron in this time. The Dark Years ended with the coming of Elendil and his sons near the end of the Second Age.
Other names of this period were the Accursed Years, the Black Years, the Days of Flight, and probably also the Dark Days.
While Robert Foster claimed that the term 'Accursed Years' referred to the same period as the 'Dark Years', Mark Fisher has noted that the former is a "term of uncertain meaning" (it could also refer to the cursed Oathbreakers, thus being a period roughly corresponding to the Third Age).