I have been juggling with the idea of a Hermetic Gruagach of House Diedne from the Schism War, turned into a troll due to Twilight while trapped in an isolated cave, who hadn't regained their senses until a couple decades ago. I hadn't noticed any rules for what such an entity looks like, so this is a tentative addition to such an NPC, with an additional caveat of losing their Intelligence (switched to Cunning) for some long period of timeVirtue Transformed Human
Qualities/Inferiorities Gigantic; FuryMy first idea for a long-scale plot for this person would be the Maker of Mages story seed from Hedge Magic; and I'm noticing that granting others Gruagach magic isn't as good as I had first thought. They know no Formulaic Spells, and I'm skeptical of their ability to learn any, so everything must be Spontaneous. On top of this, their Casting Score is fixed at 8+Stamina+Aura. This means that Give Shape and Give Visions are essentially the only spells they can cast because they have Base Effects below level 5. This is still enough to get by, but it's certainly no show-stopper. I hold trepidation with using Give Blessing to grant supernatural virtues beyond Gruagach magic and the Core Book, as I'm not sure if it's a good idea for the villain to have an army of Learned nightwalking Gruagach Magicians.
The next question is the kind of behind-the-scenes plots such a wizard-making troll would engage in, ideally ones to foster dissent and fracturing of the Order of Hermes, and to engage in greater politics with the mundanes that the Guernicus PC can engage in.
There's no reason you couldn't have him turning out Learned Nightwalking Gruagach, aside from your own sensibilities. 
You can do a lot with something like Give Shape alone, and when combined with Give Visions it makes for an exceptionally effective spymaster / assassin master and his agents.
Give Blessing is exceptionally potent without having to delve into other schools of magic. Give Premonitions, for example, can make for an amazing spy or assassin. Give Shapeshifting makes for someone able to adopt multiple disguises and, more importantly, who can simply waltz through an Aegis like it wasn't there because they're not a creature with might.
With all this capacity to spy, you get a guy who now has the means to gain access to the secrets (read: virtues) of other traditions. At which point he can start mix-and-matching; probably resulting in some horrific experiment-gone-wrong psychotic insane mages, along with his minions.
From there he has many options. Is he going to war, or is he going for the infiltration approach? Get enough of his acolytes into the Order so he can 'detonate a bomb' at Grand Tribunal? If he played that one right, he could cast the blame on a group within the Order and watch as the Order turns upon itself... unless, of course, the PCs stop him.
A minor point, but the gruagachan were bitter rivals of House Diedne. It was only after the Schism War that relationships between the gruagachan and the Order improved --- because the Order had exterminated their hated enemy. There weren't any Hermetic gruagachan before then (this is all from Hedge Magic Revised Edition).
The Contested Isle suggests that non-Hermetic Irish druids were basically reskinned gruagachan with weather control and divination. Although most of the Irish druids chose to die rather than join Diedne, some of them did not; so there could be a gruagach-like component to House Diedne.
Mark
I'm working under the assumption that the magical traditions of Diedne and Gruagach (pre-Hermetic) are functionally identical, their differences being one of political animosity. So describing the antagonist as a Hermetic Gruagach simply roughly outlines their capability, while still rightfully claiming to be a survivor of Diedne.
As for their goal, war is the primary goal, but as they are but one sorcerer, infiltration is kind of a necessity for waging it. In addition, I have to decide whether such potential (army of entrancing, learned nightwalker grugarch with shapeshifting) is legitimatley worse than what a Hermetic magus can do...create volcanoes, inevitable waves of mental domination, and innumerable other feats.
I want this NPC to be just as constrained as a PC in regards to rulings on its limits, while still being a legitmate threat by giving it sufficient power and preparation (ie, XP/time) to have spells that grant various supernatural virtues with specific prohibitions.
If you are concerned about the Order being able to easily snuff out the threat. You can simply forbid or striclty enforce some limits on the potentially most abusive mechanism (minor/major focus), ability to get easily Tractatus to progress quickly, strictly apply Integration/Original research rules (especially the need to find source of inspiration/research which can be much more lenghty and risky than the research itself).
Finally, it has been well established that given enough time, magi can be unstoppable.
So play on that: don't let them time to complete the McGuffin spell which will solve the problem, harrass their base when they are out collecting important books/elements, cut their line of supply, ruin their virtus source, send them on wild goose chase.
To be prepared, they need to know what they are facing, what kind of powers the enemy has. If their information is incomplete or wrong, their careful plan will fail.
No direct confrontation, so they cannot identify their enemy, even less vanquish him.
If the underlings of the mastermind have never seen his true form, heard his real name, all Mentem spells wont' return any useful information or even mislead them to a trap.
As long as there is no mystical link, most spell will be useless unless face to face - for the big showdown.
Mutliply also the type of threat so the PCs can never be fully prepared:
- He can have ally from another Realm (Fae, like a Troll-king), a mislead crusader with Divine power, or of course the all-too-classical diabolist or even demoniac support
- An ally with different set of power (elemental magic ?)
- Animals (immune to mind control), plants/trees
It's not that I'm concerned with Hermetics being too able to snuff the antagonist out; requiring him to be all infiltration and mastermind is fine. What I'm wondering is if the concern for my sensibilities in regards to upper potential w/Gruagach magi is misplaced. RaW, they can make learned nightwalker gruagach storm wizards on top of the more typical entrancing, fire & iron immune shapeshifters. Would a Grugach PC without real restrictions on Give Blessing be more powerful than a mountain-making mind-taker Hermetic?