For those of us that didn’t study the story in school Sir Gawain and the Green Knight tells the story of the titular Green Knight, who arrives at King Arthur’s court at Camelot with a challenge.
King Arthur’s nephew, Sir Gawain takes up the challenge, and actually beads the Green Knight, but the Green Knight then stands up laughs, picks up his head, and reminds Gawain they have a date for a year and a day from now.
The catch is that a year and a day later, the Green Knight will have the chance to return the blow.
He dares any member of King Arthur’s court to strike him with an axe.