Dain (or Dain II Ironfoot) is one of the Dwarves’ greatest warriors, was the son of Náin and grandson of Grór, the youngest son of Dáin I of Durin’s folk. Following his father’s death, Dáin became lord of the Dwarves of the Iron Hills in Wilderland; and after the death of Thorin Oakenshield and the extinction of the main branch of Durin’s line, he succeeded Thorin as King under the Mountain and King of Durin’s folk, the first not in the direct line.
Dáin’s father, Náin, was killed by the great Orc Azog during the Battle of Nanduhirion before the gates of Moria, the culmination of the War of the Dwarves and Orcs. Dáin avenged Náin, slaying Azog in single combat. He was a very young Dwarf at the time (only 32, while Dwarves reach maturity, or “battle-readiness” at the age of 30), and this was heralded as a magnificent feat. Dáin alone looked past the gate into Moria, saw that the Balrog was still active within, and knew that it was impossible for the Dwarves to return at that time. He dissuaded Thráin II from attempting to occupy Moria.
Dáin was second cousin to Thorin Oakenshield, and responded to Thorin’s call for help during the quest to reclaim Erebor. Dáin set out with several hundred Dwarves, and arrived just in time for the Battle of Five Armies (is recounted in The Hobbit). Thorin was mortally wounded in this battle, and Dáin became king after his death.