Medieval Arthurian Legend
First Continuation of Chrétien de Troyes' story of the Grail
Anonymous, late-twelfth or early-thirteenth century, Old French, Bibliothèque Nationale, Paris.
The pledge that Sir Gawain has made is to pursue the knight's quest wearing the stranger knight's own arms and riding his horse.
Sir Gawain is sent to fetch a knight back to Queen Guinevere's pavilion. The knight protests that he is on an urgent mission and cannot delay. Sir Gawain agrees to take up the knight's quest, should the delay compromise the knight's mission, and only upon this promise does the knight return with Sir Gawain.
The pledge that Sir Gawain has made, in fact, is to pursue the knight's quest wearing the stranger knight's own arms and riding his horse. And when this knight is killed unexpectedly, he holds Sir Gawain to his promise before he dies.
So take my arms right away, and arm yourself and mount my horse, for he'll lead you directly without fail, on the great mission that I was to have carried out.