Final answer:
The Green Knight is the pivotal character who enters Camelot and challenges the Knights of the Round Table in 'Sir Gawain and the Green Knight', a tale of chivalry and honor.
Step-by-step explanation:
A pivotal moment in Sir Gawain and the Green Knight is when the Green Knight enters Camelot and challenges King Arthur and the Knights of the Round Table. The Green Knight's challenge sets off a sequence of events that test the honor and chivalry of Sir Gawain, one of the noble knights of the Round Table. The text you've provided includes various references to Camelot, knights, and legendary figures associated with the Arthurian romances, highlighting the chivalric and romantic ideals that are intertwined with the story of Sir Gawain and the Green Knight.