In the assault of the Dagor Bragollach, the Men of Dorthonion were overrun by Morgoth's forces. Their lord, Barahir, took the survivors of his House into hiding in the northern highlands, above the pine forests of that region, but they were harried and hunted by Morgoth's servant Sauron, and their numbers dwindled.
At last, only thirteen Men remained, and Ragnor was one of these: a faithful servant and soldier of the House of Bëor. One fateful night, as the outlaws slept by the shores of a mountain lake, they were set upon by Orcs, and the entire band was slain. Beren was absent from the camp, and survived, but Ragnor and all his companions were lost.
Notes
1
The Etymologies gives us an adjective, ragna, that means 'crooked', but there's no direct evidence to link this with Ragnor's name. (The History of Middle-earth volume V, The Lost Road and Other Writings III The Etymologies).