When Isildur Elendilion claimed the Ring for his own--he was ambushed at the Gladden Fields by a horde of orcs from the Misty Mountains. The result was that Isildur and his sons were slain and the Ring was lost down the Anduin River. This event is what allowed for Sauron to return and gain power again in the world. Ohtar of Arnor, a servant of Isildur, took the shards of Narsil to Imladris for safe keeping.
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