Warning: Wikipedia contains spoilers
Náin is the name of two Kings of Durin's Folk.
Náin I was the son of Durin VI, and succeeded his father as King of Khazad-dûm. When his father was slain by the Balrog, Náin I attempted to destroy it, but was himself killed the following year. The remainder of Durin's Folk fled Khazad-dûm, and renamed it Moria.
Náin II was another King of Durin's Folk, son of Óin. Under Náin II, the Dwarves lived in the Grey Mountains, until they were attacked by dragonss. He was succeeded by his son, Dáin I.