Thrόr was the grandfather of Thorin Oakenshield. Thrόr lead a contingent of his people to Erebor, re-establishing the Kingdom Under the Mountain that had been abandoned by his forefathers years before. King Thrόr and the people of Dale became fabulously wealthy, and renowned throughout middle-earth for their crafts. As their reputation for riches and the creation of fine artifacts spread, it finally reached the ears of Smaug, the dragon. Craving the gold and jewels of the dwarves, Smaug attacked the city of Dale and the Lonely Mountain in the year T.A. 2770, driving the dwarves into the wilderness. Many of his people were killed. However, Thrόr, his son Thráin, and his grandson Thorin escaped.
According to what Gandalf told Thorin at the Unexpected Party at Bilbo’s house, It was Thror who made the map of the Lonely Mountain and surrounding area. The location of the secret door in the mountain would eventually allow Bilbo to enter a secret passage in the side of the mountain that would lead him to Smaug’s lair.