Most famously found in Sir Gawain and the Green Knight, the Beheading Game typically involves a challenge to a knight. The challenger proposes that the hero strike a blow with an axe to the challenger's head; but the hero has to promise to go to a designated location at the end of a period of time to receive a reciprocal blow. Figuring that if his own strike is accurate enough, he won't have to uphold his end of the promise, the hero accepts the challenge and cleanly beheads the challenger. The hero is astonished, however, when the decapitated knight picks up his head and walks away, disappears, or otherwise suggests that he is not dead and that some enchantment is involved.
The poor hero must now go to his certain death or break his vow. Choosing death over dishonor, the hero appears at the designated location, at the designated times. After perhaps a feinted blow or two, the hero is granted a reprieve because he has acquitted himself with honor.
In Sir Gawain and the Green Knight and its inferior successor, The Grene Knight, the Green Knight, in the service of Morgan le Fay, is the challenger, and Gawain is the hero. This theme is used several times prior to Gawain in the First Continuation of Chrétien's Perceval, in which Caradoc is the hero and the challenger is his own father, Eliavres; in Perlesvaus, in which Lancelot is the hero, and his honorable return saves not only his own life, but also restores the Waste City to prosperity; in La Mule sans frein, in which Gawain is challenged to the test by a churl; and in Heinrich von dem Türlin's Diu Crône (in an episode based on La Mule), in which Gawain's participation in a beheading game against the sorcerer Gansguoter is a condition of his retrieving a magic bridle for the lady Sgoidamur.
Probably the most humorous of the pre-Gawain beheading games occurs in Hunbaut. Gawain, challenged to the contest by a churl, decapitates his opponent with an axe. Gawain, apparently familiar with how the game usually goes, then grabs the churl's body and holds it fast, so that, unable to pick up and refasten his head, the churl dies.
We find a variation in the Middle-English The Turke and Gawain. A churl arrives at Arthur's court and offers a non-fatal version of the Beheading Game with fists instead of axes. After allowing Gawain to strike him, the churl insists on delivering the reciprocal blow at a later time, and he leads Gawain on a series of adventures. Finally, on the Isle of Man, the churl insists that Gawain strike him again, but this time with an axe. When Gawain beheads the churl, his companion is released from a curse and becomes the noble Sir Gromer. The Carle of Carlisle includes a similar scene in which Gawain's beheading of a bewitched churl transforms him into a nobleman.
The theme ultimately originates in Irish non-Arthurian romance. A tale called Bricriu's Feast (c. 1100) contains an episode between the hero Cuchulainn (often seen as the Irish counterpart to Gawain) and a sorcerer that was apparently the inspiration for the test in Sir Gawain.
Gawaine - Beheading Contest | The Legend of King Arthur