In a new video released by the BBC, the Irish actor insisted that the fantasy drama has "definitely - after five years - run its natural course".
"It's come to its highest point - it's grown and grown each year," he said. "It has become darker, it has become stronger, characters have grown up, the audience has grown up with it and... we want to end on its strongest point. I think we've achieved that."
Morgan also hinted at the show's conclusion, suggesting that the fifth and final series of Merlin is charged with a feeling of "impending doom".
"Merlin is shown that Arthur (Bradley James) will die at Mordred's (Alexander Vlahos) hand," he explained. "That hangs over his head, so as a result he has become a little bit more ruthless, a little bit more focused, and a lot more grown-up and a lot more adult in his decisions.
"But I think there's a part of him that knows it's coming and that he'll try all he can to prevent it, but there's greater things at work."
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In a final message to Merlin fans, the 26-year-old expressed his desire to send out a "massive thank you".
"The support and the absolute encouragement that we've received from people has just been off the scale," he said.
Merlin will conclude this Christmas with a two-part series finale.
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