Battle of DagorladBattle of Dagorlad (3434 S.A.) - The battle before the battle.  It was fought outside Mordor and was a precursor to the invasion of the Black Land.  By the Third Age the area had become known as the Dead Marshes as a result of the corpses left over from the battle.

 

"'There was a great battle long ago, yes so they told him when Sméagol was young, when I was young before the Precious came.  It was a great battle.  Tall Men with long swords, and terrible Elves, and Orcses shrieking.  They fought on the plain for days and months at the Black Gates.'" ~The Two Towers, Book IV, chap. 2 (The Passage of the Marshes)

 

It was at Dagorlad that Oropher, leader of the Woodland Realm, was killed.

 

"Oropher was slain in the first assault upon Morder, rushing forward at the head of the most doughty warriors before Gil-galad had given the signal for the advance.  Thranduil, his son survived (and became the new King of the Woodland Realm)." ~Unfinished Tales (The History of Galadriel and Celeborn)