I've always assumed that the big scary-looking wizard in the red robes, shown on the game screen, is Morcar/Zargon. (The bazillion monsters he's directing forward kinda give it away.)
Meanwhile, I always thought Mentor was the wizardly fellow shown poring over a book (Loretome?) in that little window on the European Heroes' character cards.
Believe it or not, it's taken me twenty years to notice that they're identical.
Now, I reckon the real explanation for this is simply that both pictures show Morcar/Zargon, and that there's no picture of Mentor. Possibly the little picture on the character cards was removed from the North American version so nobody would mistake it for Mentor.
HOWEVER... just for fun
... what if Mentor and Morcar are the same person? A case of split personalities? Curious, don't you think, that Mentor's Loretome seems to know all about Morcar's schemes, yet neither seems capable of truly defeating the other? What if Mentor leads a double life as Morcar and neither of them realises it? What if all that Morcar backstory Mentor tells the heroes in the first questbook is nothing but lies and/or fantasies he's built up to protect himself from the horrifying truth? It wasn't his apprentice who 'saw magic as a shortcut to power'. It was himself. Dun dun dunnnn...
It's certainly suspicious that their names share the same first and last letters...
Psychologists, to arms!
Of course, the Marvel Winter Special muddies the waters by showing Morcar/Zargon as a hooded blue-skinned fellow. I propose to deal with the problem thusly: Not canon.
From a meta perspective, the Morcar/Zargon player has the dual role of trying to defeat the Heroes and giving them an enjoyable, fulfilling game. There's a certain about of doublethink involved. It certainly feels like you're Mentor and Morcar simultaneously
And from a fan Quest Pack perspective, imagine this: The Heroes are summoned by a strangely flustered and panicky Mentor because he's made some terrible discovery. They find him gone and a mysterious magic portal open in his study, leading into the heart of Morcar's bastion. The Heroes assume Mentor has been kidnapped and leap through the portal to save him, fighting through the toughest Quests they've ever attempted, perhaps even confronting Morcar himself... but all the while they notice increasingly weird coincidences and connections, and slowly begin to realise the truth... Mentor and Morcar are never in the same room at the same time...
) my hapless band of heroes will realize that by defeating Morcar and Zargon, they sent the repressed power back into Mentor; but now unfettered by his control, unbalancing Mentor and sending him warring within himself, his power feeding back onto the physical plane. It borrows a bit from the Prof. X and Onslaught interaction, and ultimately the heroes will have to slay a crazed Mentor at his request before seeking out a greater avatar of Xadius created by Morcar and Zargon pooling power gained outside of siphoning Mentor.
Tony Curtis, Ernest Borgnine, et al, wasn't (Sir) Ragnar one of the Vikings 
