One Great Leap
The Music never stopped for those who had the ability to hear it, the harmonies and rhythms familiar as the Ages rolled past. On rare occasions there was a change in the world, and a change in the Music. But that had not happened for a very long time...
But this day there was a change. Was it a new theme? No, not new; rather a variation on a theme heard only once and that fleetingly, in the Music created by the One, a small variation in the song of the Secondborn of His Children, light and sprightly. But now that variation was swelling into something new.
In the West, the Ainur listened, entranced, but not understanding. All of the Ainur save two: Yavanna Kementári and Nienna looked at one another and understood that a secret known to none other save Eru Ilúvatar, who had entrusted them with it, was about to be revealed. A seed long planted was about to come to fruition.
Far away they felt it, a small heart fluttering in terror, trapped in the dark. But something else drove out the terror, a mighty wave of sorrow and compassion, followed by steely determination.
And now all the Ainur could feel it: one small being of a kind not yet known by them had made a decision that would alter the fate of Arda.
He trembled. And then quite suddenly in another flash, as if lifted by a new strength and resolve, he leaped.
No great leap for a man, but a leap in the dark. Straight over Gollum's head he jumped, seven feet forward and three in the air: indeed had he known it, he only just missed cracking his skull on the low arch of the passage...*
The Music swelled in triumph!
Manwë summoned Eönwë: "Call your people, send word to the Great Eagles. Their intervention will be needed."
All unknowing, Bilbo Baggins burst forth from beneath the mountain. He left his buttons behind, and carried the fate of Arda in his pocket.
*From The Hobbit, Chapter V, "Riddles in the Dark"
Author's Notes: This makes use of a bit of fanon from my story "Ancestress" in which Eru brings forth the hobbit race (among the Secondborn through their common fate of mortality) secretly, and asks Yavanna to see to their guarding and Nienna to gift them with extra compassion. But the hobbits are kept secret from everyone else.
I also make a reference to a bit of fanon from pandemonium_213's and my co-authored story "The Prisoner and the Hobbit", in which the Great Eagles are the people of Eönwë. In that story, Bilbo learns an unexpected fact about the Eagles coming to the rescue.