According to Tolkien, a eucatastrophe in a story often happens at the darkest moment. When all seems lost – when the enemy seems to have won – a sudden “joyous turn” for the better can emerge. It delivers a deep emotional reaction in readers: “a catch of the breath, a beat and lifting of the heart”, he wrote.
In The Hobbit, it’d be the sudden arrival of the eagles in the Battle of the Five Armies, while in The Lord of the Rings, it’s the moment Gollum unexpectedly falls into the cracks of Mount Doom, destroying the One Ring. But many other stories feature such turning points, whether it is the kiss that revives Snow White, or the destruction of the Death Star in Star Wars.
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https://www.bbc.com/future/article/20221005-eucatastrophe-tolkiens-word-for-the-anti-doomsday