One of those stray thoughts that one has from time to time (me, in this case) has made me think that I do not recall Merlin performing any magic in the Arthurian stories I recall. He acts like a counselor, but does not create items (he gets them from the fae) or cast spells. He seems wise, not magically powerful. The only spell I recall him casting is a MuCo spell on Uthred so he can conceive Arthur. Looking at what he does in the stories I am aware of, I got up the following list
Prophecies & preternatural knowledge of past & future. Visions can provide this knowledge
Transform Uthred so he can enter Tintagel. Now this is a spell
Negotiate with the lady of the lake to get a few trinkets. Faerie Lore for sure, maybe summoning, if we stretch it, but that is it.
Give advise to Arthur. That looks like a fairly high Area Lore, Folk Ken, Intrigue and maybe Artes liberales (inceptions), but no magic involved
Nimue/Viviane enchants merlin with his own abilities. Those include Rego-like spells that force Merlin to do her bidding. Those powers are never seen in use in the hands of Merlin, IIRC.
Robert de Boron (in his prose "Merlin") makes him able to shapeshift, but in later (Cretien de Troyes) stories that is not an ability he uses at all. In this prose he is also quite a herder, so Animal Ken might work
Now, hit me with a brick and tell me the wonderous spells that merlin did cast I am trying to figure the kind of powers that Merlin would have had in an Ars setting (and I am sure several seekers of the ooH will have done that for sure). If he was weak I wonder why the OoH places such emphasy on him. For my actual reading it seems that he is way weaker than any of the traditions of the Founders and that doesn't seem "right".... even if he is an actual legend, while flambeau isn't outside ME.
After reading HMRE just recently I got the idea thar Merlin might have been a powerful gruagach.
Shape changing: Merlin transformed the shape of Uther, so he could enter Tintagel. Merlin was also said to be a shape changer himself. A gruagach could do this with give shape spells. Visions: Somewhere in HMRE it is stated that powerful gruagach can perceive the future. Other magics: gruagach cast blessings and geas. This spells are not very flashy, so it might seem to the storytellers that Merlin never apparently cast a single spell.
HMRE ist my latest purchase, so I don't know the rules very well. But when I read the chapter about gruagach for the first time, the name "Merlin" just crossed my mind.
Perhaps Merlyn was simply too canny, too crafty and too careful to go around letting everybody and his dog see him casting spells. The philosophy being: the more everyone sees me the more they know about me, the less they see me the more their imagination grants me powers (I may or may not have). This is why the Navy SEALs are mum about 99% of their operations...it creates mystique.
The most powerful magic is the least flashy (think Gandalf from LotR books, not movie). Merlin by will alone returned magic to Britain. His willpower elevated Uther and then Arthur to the Throne of Britain. He bent the faerie of Avalon to his will, forcing them to give up Excalibur for a mere mortal.
As an aside, he was also half demon, the son of the Devil, and experienced time going backwards.
Did he? I was unaware of that "Supreme Willpower" thing. Was magic out of the island before? The throne thing is not so clear to me. it looks like he was a skilled diplomat and manipulator, not necessary magic use there.
Was the case of the lady of the lake so forceful? I thought it was more a bargained settlement, but I am not that proficient in arthurian legends
Yeah, it's really a question of who and what do you want Merlin to be. He could be the Son of the Devil, who lived backwards. He could be the last Grand Druid. He could be a prophetic madman who lived in the woods. He could be the greatest magus who ever lived, or a total fraud, or anything in between.
I don't consider Geoffrey on Monmouth's take on the history of Britain to be particularly cool, and I haven't yet been able to come up with anything cool derived from it. In my sagas I like to keep whatever was really going on with King Arthur, Merlin and all that early Dark Ages stuff extremely vague - the PCs might go off into Arcadia and meet half a dozen Merlins that are all totally different from each other, and yet learn nothing about what the real one was actually like.