Sire Gauvain et le Chevalier Vert

A mysterious Green Knight rides into a court celebration and challenges the young knights present to strike him with his own axe - on the condition that he, the Green Knight, may return exactly the same blow in a year and a day. Gawain accepts and cuts off his head. But the Green Knight stands up, retrieves his head, and rides away, reminding Gawain of his promise.