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In the year S.A. 3434, the host of the Last Alliance of Elves and Men fought the forces of Mordor in the Battle of Dagorlad. During the battle on the plains, more than half of the Elves of Lothlórien under the command of King Malgalad were driven into the Dead Marshes. After the battle, many of the slain were buried outside of the marshy area, but over time (in the Third Age) the Marshes had grown and swallowed the graves.
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In T.A. 1944, King Ondoher of Gondor was caught by a surprise attack by the Wainriders upon the Dagorlad. When the King and his guard were destroyed, many of the confused soldiers of Gondor were driven by the attackers into the Dead Marshes. However, after Eärnil II won the Battle of the Camp, those of the Wainriders who were not slain in the fight were themselves driven into the Dead Marshes and there perished.
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