The father-son relationship between Thrain and Thorin Oakenshield

Thrain, son of Thror, was the father of Thorin Oakenshield, and their relationship is one of the foundational losses that drive the entire quest of The Hobbit. Thrain was the heir to the Kingdom under the Mountain after the dragon Smaug destroyed the dwarf-realm of Dale and the Lonely Mountain. He and his father Thror escaped the dragon's attack with singed beards, but they were driven into exile, and the family was scattered. Thrain's fate after the fall of the Mountain became a mystery that haunted Thorin for a century.

Separation and the Map

Thrain's disappearance is a pivotal event in Thorin's life. According to Gandalf, Thrain went away on the twenty-first of April, a hundred years before the events of the story, and was never seen by Thorin again. Before he vanished, Thrain gave the map of the Mountain and the key to the secret door to Gandalf, entrusting the wizard with the task of delivering them to his son. Thrain was unable to remember his own name when he gave Gandalf the paper, and he never told the wizard Thorin's name, making the delivery a long and difficult task. This act of entrusting the map and key to a third party, rather than to his son directly, underscores the desperate and broken state of the dwarf-king in his final days.

Imprisonment and Death

Gandalf later revealed that he found Thrain a prisoner in the dungeons of the Necromancer. The wizard tried to save him, but it was too late; Thrain was witless and wandering, and had forgotten almost everything except the map and the key. This revelation, made during the Unexpected Party at Bag-End, filled Thorin and the other dwarves with a shudder of horror, and Thorin vowed to give a thought to the Necromancer. Gandalf, however, dismissed this, stating that the Necromancer was an enemy far beyond the powers of all the dwarves put together, and that the dragon and the Mountain were more than big enough tasks for Thorin. Thrain's imprisonment and death at the hands of the Necromancer thus became a secondary, unavenged wound in Thorin's history, adding a layer of personal grievance to the already monumental quest for the dragon's treasure.

Legacy and the Quest

Thrain's legacy is the driving force of Thorin's character. The map and key, which Thrain had preserved even in his madness, became the central instruments of the quest. Thorin's identity as the heir of Thrain and Thror, the King under the Mountain, is repeatedly asserted throughout the story. When he announces himself in Lake-town, he declares, "I am Thorin son of Thrain son of Thror King under the Mountain!" This lineage is his claim to the treasure and his authority. The memory of his father's escape and the lost kingdom fuels Thorin's pride, his determination, and ultimately his tragic flaw—the dragon-sickness that grips him when the treasure is within reach. The relationship between Thrain and Thorin is thus one of inheritance not only of a kingdom but also of a burden of loss, a quest for restoration, and a deep, unresolved grief that shapes the entire adventure.