The 12 Works of Hercules.
After murdering his wife and children in a time of insanity. he wanders through the streets of Greece, until one day he finds an oracle, which guides him.
To be redeemed from his guilt, he should do 12 tasks. And the list of such impossible tasks would be given by his cousin, king of Mycenae and Tiryns.

King Arthur and the Knights of the Round Table.
Arthur, after establishing peace in Britain, summoned honorable knights to establish his entourage.
And he desired a room where justice prevailed, and all were his equal. And, according to a legend, the question was solved.
The solution was found by a carpenter named Cornish, who designed a rounded flat.