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The Gruagach was not Mogdred.

He meets the same description.
And it say he has many different names.
Mogdread has also been described on the show as a sorcerer

Unless there is more to the story in other books I haven't read yet.
But it seems that's who they were implying it was.

The Gruagach has many names because he's the Devil... Mogdred might be evil but I don't think he's quite up there (or down there, if you prefer).

The background to Mogdred is quite clearly established in the series. He is Merlin's evil half. There is nothing in the books that even hints at the Gruagach having any links to Merlin at all, even though Merlin appears in Treguard's dream and discusses the Gruagach with him in some detail. The implication from the scene where Treguard and Folly enter Dunshelm is that the Gruagach may be an incarnation of the Devil.

And the physical description in the book only bears a passing resemblance to Mogdred; -

"The face was as white as parchment and, although the features were regular, the combination of thin triangular jaw, the slash of the mouth, the beaked nose and bulging dome of the forehead - all these left an impression of something utterly alien." It also mentions that the Gruagach had eyes that "burned with a living green fire".

The only real point of resemblance is the whiteness of the skin. Thin, triangular jaw? No, John Woodnutt's face was quite rounded, and his jaw was fairly broad. Slash of a mouth? No. I'm not suggesting Woodnutt had a big mouth, but it was wide, very animated and flexible. Beaked nose? Not really. Woodnutt's nose had a flattened tip, it was not beaked. Bulging forehead? Well, Woodnutt's balding head gave an impression of a domed forehead, but it was hardly bulging. Green eyes? No, Mogdred's eyes were dark brown, bordering on pitch black.

Also, the Gruagach died at the end of the first book, a fact that was reiterated in several of the later books, without any suggestion of a resurrection.

And there were plenty sorcerers in KM, Wombstar. By using that as an indicator, you could just as easily conclude that Grimaldine was the Gruagach.

Well it's not for certain and there is no right or wrong answer.
We're all entitled to our own interpretation.

It's also said only his physical form was killed.
And the book is not going to be to 100% the same. Folly in the book is nothing like it tv counterpart, other than his name and profession his personality is more like Motley in the book, he also talks about being a former wizard.
So the book has to take some liberties to suit itself.

I totally agree Wombstar.