Born in T.A. 2450, Bórin was the younger son of King Náin II. During his youth, the House of Durin lived in the dragon-plagued Grey Mountains that were ruled by his brother Dáin I. When the King and his son Frór were slain by a Cold-drake, Durin's folk divided. Bórin followed the new King Thrór back to the Lonely Mountain, while others followed Grór to the Iron Hills. Bórin lived with the Dwarves of Erebor under King Thrór for the rest of his life.