Eucatastrophe

Eucatastrophe is a neologism coined by Tolkien from Greek ευ- "good" and καταστροφή "sudden turn".
In essence, a eucatastrohpe is a massive turn in fortune from a seemingly unconquerable situation to an unforseen victory, usually brought by grace rather than heroic effort. Such a turn is catastrophic in the sense of its breadth and surprise and positive in that a great evil or misfortune is averted.
Tolkien himself made use of the trope of eucatastrophe several times in his own writings. The most famous usage is the final destruction of the One Ring within Mount Doom. Frodo was unable to destroy the Ring by his own strength. The Ring is destroyed by the intervention of Ilúvatar after it is taken from Frodo by Gollum, who Frodo himself spared earlier in the story. Had Gollum not been there, the Ring may only have been destroyed by Frodo casting himself into the fire, if it was destroyed at all.
The other famous example of a eucatastrophe is the War of Wrath fought on behalf of the Eldar at the end of the First Age. Eärendil was able to reach Valinor and persuade the Valar to go to war against Morgoth for the sake of Elves and Men. Without the intervention of the Valar, Morgoth would never have been defeated by any mortal strength.