This is a reference to the Middle English poem written as part of the alliterative revival in the late 14th century . A Green Knight enters Arthur's hall and dares Arthur's knights to chop off his head, on the condition that the Green Knight be allowed to do the same to the knight a year later. Gawain volunteers and chops off the Green Knight's head. But the Green Knight magically survives the blow. Gawain's challenge in this poem is to have the courage to keep his pledge to the Green Knight the following year.