"Find the keyhole!"
They couldn't find the keyhole. There was no keyhole. And the light was failing.
"Break it down!" They attacked the wall with everything they had but Thorin knew it wouldn't work. He ignored it though. How could he turn back now? How could he ever return to the Blue Mountains and admit that Erebor had refused to let its King in? By Mahal, how could he ever look his nephews in the eyes again? But Balin's words broke through his desperation and all was silent. He lost all dignity and began begging, begging to know what they had missed. But the truth rang out.
They had lost the light.
And so he turned away, from his Fathers, his destiny, his home. A failure.
"You can't give up now!" The earnest voice of their Hobbit rang out. If Thorin had any sort of energy he would've smiled. That was their Hobbit, always refusing to give up, even if the fight was not his. His despair came again. They had taken their Hobbit so far from his precious home, almost got him killed so many times, and for what? Nothing.
The Key dropped from his hand and he watched it fall. Watched it lay there, useless on the ground, and left it there. Thorin felt as if he were underwater, the world passed by him in a muffled haze. He shoved the map into Bilbo's chest. The Hobbit liked maps, didn't he? Perhaps he would keep it as a memento.
He shuffled, his feet lead, not willing to climb back down the stairs he had climbed up with such hope.
Until … "Come back! Come back! It's the light of the moon, the last moon of autumn!" He turned back and for a moment just watched his beautiful Burglar searching for the Key. It seemed Master Baggins had saved them once again.
Thorin turned the Key, felt, rather than saw, their Burglar gearing himself up beside him. It was time for their Burglar to truly become a Burglar and Thorin gave him a reassuring nod before he entered Erebor.
The memories came like a crashing wave. The corridors he had wandered in his youth, the halls filled with golden light, the glorious feasts filled with food and song. He allowed his hand to feel the stone he knew so well. He dreamed of the halls being filled with song again, of the Dwarves returning home.
And of the legendary treasure that lay within.