He is most remembered as the elf whose murder initiated the Nírnaeth Arnoediad (Battle of Unnumbered Tears).
Gelmir was a Noldor elf from Nargothrond and the brother of Gwindor. Gelmir was captured during the Dagor Bragollach (Battle of Sudden Flame) and brought to Angband where he was tortured and blinded. He was enslaved there for seventeen years until the Nírnaeth Arnoediad where he was executed.
Before fighting broke out, Morgoth used him as bait to draw out the Orodreth's forces. His orcs dragged him in front the army of Nargothrond so that his people could see him die a horrible death. The orcs chopped off his arms and legs and left him to die slowly of his wounds within sight of his of loved ones. Gwindor and his fellow soldiers were so enraged by this act that they charged after them too soon putting Morgoth's forces at an advantage.