Girnir was the son of Lord Girion who had survived a terrible rainstorm that was casted by Smaug. Seeking refuge in Esgaroth, Girnir had many descendants that lived along the long lake. After his father's death, Girnir managed to claim the last Black Arrow that his father failed to use, and kept it hidden in Esgaroth until one of his descendants Bard Bowman used it to kill Smaug.