Keep in mind, though, it hasn't been confirmed yet.
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A little disappointing that the role would be recast since the boy who played him in S1 and S2 was brilliant, but with the time jumps between seasons and the seasons themselves covering a fair amount of time, it makes sense to recast Mordred. Besides, Asa Butterfield is only 14 or 15 years old now. Probably a little young to play a teen Mordred, even though the character's age would likely be somewhere from 14 to 17.