In King Arthur's Knights, what happened to the knights who went in search of the Holy Grail?

They most likely died.

Explanation:

In King Arthur's Knights, we learn that many knights tried to go in pursuit of the Holy Grail. These knights accompanied Sir Galahad, who is the only one considered worthy enough to find it. The group separated, as they believed this would make their mission easier. Although we do not learn what happened to the other knights, it is implied that they all died. It is only Galahad, Bors and Percival who were able to continue the journey.

When Lancelot returned to Camelot, the knights were all summoned to meet at the Round Table. As he took his seat, he saw that there was now an inscription above the Siege Perilous.

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