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During the War of the Ring, Riodur was among the elves who accompanied Haldor of Lorien to Helm's Deep, a large valley in the north-western Ered Nimrais. There, they aided King Theoden and his people in defending the keep from an army of 10,000 Uruk-hai that were dispatched by Saruman to destroy the people of Rohan. He was stationed at the Deeping Wall and fought in the Battle of Helm's Deep. As the Uruk-hai were charging towards the fortress, including the Deeping Wall, Riodur and the elves loaded their bows and demonstrated their unmatched proficiency of archery, as they loosened devastating hails of arrows in seconds, killing many Uruk-hai. Riodur slew a large number of Uruk-hai during the battle. While Riodur and the elves were busy sword-fighting with the Uruk-hai on the wall, a group of Uruk-hai sappers laid the charges that were prepared by Saruman in the culvert under the wall. As the berserker lit the blasting powder, despite Legolas' repeated attempts to bring him down, Riodur was killed in the ensuing explosion.
- Riodur is not seen in the books.
- He is featured in the Decipher Trading card game.