One of the band of men who remained and fought beside Barahir and Beren after the land of Dorthonion was overwhelmed by Morgoth. Dairuin was one of the last twelve survivors of Barahir's men, whose deeds in defiance of the occupying armies provoked Morgoth into action against them. The outlaws were betrayed by Gorlim the Unhappy, and Dairuin was slain with almost all the others by the shores of Tarn Aeluin. Only Beren, who was away from the outlaw camp at the time, survived the massacre.
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Dairuin's name is never explicitly explained. It seems to contain elements that could be interpreted as 'shadow flame', though if this is correct, the significance of such a striking image remains unknown. The -ruin element, literally 'red flame', may actually be a family signifier, since there was another member of the same outlaw group named Radhruin, whose naming order among the group implies that he may have been Dairuin's brother.