The subject of this article originates from non-canonical sources. To find out about what is considered "canon" see LOTR:Canon.
During the time when the War of the Ring began, Lhunurion was among the elves that refused to leave Middle-earth and preferred to fight against the enemies of Mordor. Unlike the War of the Last Alliance that happened at the final year of the Second Age, Lhunurion accompanied Haldir of Lórien to the old fortress of Helm's Deep where they aided King Théoden and his small army in defending the fortress of Saruman's army of 10,000 Uruk-hai that were tasked by their leader to destroy the people of Rohan. To prevent such atrocity, Lhunurion and his kin were stationed on top of the Deeping Wall. He was standing behind Legolas and Gimli, looking onwards at the arrival of Saruman's army. As the Uruk-hai prepared to lay siege, Lhunurion loaded his bow and killed a number of Uruks throughout the battle. However, he was among the elves that were killed, but his life along with those that fought and defended the fortress would be honored by King Théoden for defending Rohan and its people.
He was not a character invented by Tolkien.