The Twelve Dancing Princesses

A king, baffled that his twelve daughters' dancing shoes were worn through every single morning despite being locked in their bedroom each night, offered his kingdom and his choice of daughter's hand to any man who could solve the mystery — with the warning that any who failed within three nights would be executed. Prince after prince tried and failed, falling into an unexplained deep sleep each night, unable to see what happened.

A poor, weary soldier, given a magical cloak of invisibility and warned never to drink the wine offered by the princesses, only pretended to drink it and stayed awake, watching in secret. He saw the princesses open a hidden trapdoor and descend a magical staircase into an underground world, where twelve princes waited to row them across a lake to a glittering ballroom, dancing until dawn wore their shoes through completely.

The soldier followed invisibly each night, gathering proof — a branch from a silver tree, a golden goblet from the ball — and on the third day revealed everything to the king. The princesses' secret nightly escape was ended, the enchanted princes banished, and the soldier, choosing the eldest princess as his reward, was married and made heir to the kingdom.