Final answer:
In 'Sir Gawain and the Green Knight,' the frightening moment comes when the Green Knight, despite being beheaded, picks up his head and speaks to Gawain.
Step-by-step explanation:
A frightening part of Sir Gawain and the Green Knight is when the Green Knight, who should be dead, picks up his own severed head and speaks to Gawain, warning him that in one year's time, he must seek out the Green Knight to receive a blow in return. This supernatural event is typical of the Middle English chivalric romance genre, mixing elements of magic and knighthood. In this tale, the resilience of the Green Knight after Gawain beheads him at King Arthur's court is both shocking and mystifying, setting the stage for Gawain's subsequent moral tests and adventures.