The father-son relationship between Thror and Thrain

Thror and Thrain were the father and son who ruled as the Kings under the Mountain in the Lonely Mountain before the coming of the dragon Smaug. Their relationship is the dynastic foundation of the entire quest: it is Thror who commissioned the map that leads the company to the secret door, and Thrain who passed both the map and the key to Gandalf after his own imprisonment. The bond between them is defined by shared catastrophe, a lost kingdom, and a legacy of vengeance that Thorin Oakenshield inherits.

Identity and Royal Lineage

Thror was the King under the Mountain, the grandfather of Thorin Oakenshield. He was the son of Thrain the Old, the dwarf who first discovered the Lonely Mountain. Thror's reign saw the dwarves mine and tunnel the mountain into vast halls and workshops, amassing immense wealth in gold and jewels. Thrain was Thror's son and the father of Thorin. After Thror's death, Thrain became the rightful heir to the throne, but he never reclaimed it. The map that Gandalf later gives to Thorin was made by Thror, and it was Thror who gave the map to his son Thrain for safekeeping before he went to the mines of Moria.

The Fall of the Kingdom and the Escape

When Smaug attacked the Lonely Mountain, Thror and Thrain were among the few dwarves who escaped. Thorin recounts that he was unexpectedly joined by his father and grandfather, both with singed beards, after the dragon's assault. They looked grim but said very little, and when Thorin asked how they had gotten away, they told him to hold his tongue, saying that one day in the proper time he should know. This secrecy suggests that Thror and Thrain knew of a private side-door that only they knew about, a secret that later becomes central to the quest. Thror was later killed in the mines of Moria by Azog the Goblin. Thrain then went away on the twenty-first of April, a hundred years before the events of the story, and was never seen by Thorin again.

The Map and the Key

The map of the Mountain, made by Thror, was given to his son Thrain for safety before Thror went to Moria. Thrain, after his father's death, went away to try his luck with the map but never got near the Mountain. He had many unpleasant adventures and was eventually found by Gandalf as a prisoner in the dungeons of the Necromancer. By that time, Thrain was witless and wandering, and had forgotten almost everything except the map and the key. He gave the map to Gandalf to give to Thorin, and the key was also handed over. This act of passing the map and key from father to son, and then through Gandalf to Thorin, is the direct link that enables the quest to find the secret door and enter the Mountain.

Legacy and Significance

The relationship between Thror and Thrain is defined by loss and the transmission of a legacy of revenge. Their kingdom was destroyed, their treasure stolen, and both died without reclaiming it. Thror was killed in Moria, and Thrain died a prisoner of the Necromancer, his mind broken. Yet the map and key they preserved became the instruments of their descendant's attempt to reclaim the lost kingdom. The story of their escape and the secret they kept is the hidden history that drives the plot, and their fate underscores the cost of the dragon's depredations and the long, bitter exile of the dwarves of the Line of Durin.