Haudh-en-Nirnaeth is the Elvish name for 'Hill of Tears'. It was also named Haudh-en-Ndengin, which, in Elvish, is translated as 'the 'Hill of the Slain'.
This "hill" was not a geographical formation, but rather a gigantic mound composed of all the corpses and weapons of the Elves and Edain that fell during Nirnaeth Arnoediad, the Battle of Unnumbered Tears. It is located in Anfauglith/Ard-Galen/'The Gasping Dust'. After the battle, Morgoth commanded his creatures to build the mound as a gruesome monument to the failure of the Elves during the battle. But in spite of his intention, the mound quickly became covered by green grass despite existing in the middle of a desert, and no servant of Morgoth Bauglir ever set foot on it again.