While our troupe has gone through some fits and starts, we are still planning to play in Scotland. None of the current players read this forum (for shame) so I am not worried about spoilers.
The overarching plot that I see is the return of Damhan-Allaidh (why mess with the tried and true). Because I want to set this plot up early without overpowering the young magi or diminishing a threat as powerful as the Spider, I want to set up early encounters with supporters and plotters. The encounters will grow more dangerous as the magi get closer to the truth about the real threat.
The Lion in the North offers some good stuff, but the magic needs to be reconfigured for Fifth Ed. Moreover, RoP:I and HoH:S both describe the Ex-Misc Damhadh-Duidsan as descendents of the same tradition as Damhan-Allaidh, which leads me to believe that their powers should be similar. He also had supported among the gruagachan according to Hedge Magic, which may or may not be a departure from Lion in the North. We also know that he was powerful enough to defeat the order in Carnac and in the order's first expedition into Britain. He appears to be able to cause considerable destruction, call the Sluggh (I want to keep this power into 5thEd), have infernal powers (Damhadh-Duidsan), and hide his soul for near- immortality (gruagachan). He also performs human sacrifice (Incantation?) is said to be descended of Giants like the Damhadh-Duidsan and gruagachan, and likely was a shape changer (maybe through his giant blood).
A while back, someone posted a comparison of Damhan-Allaidh to the villain in Harry Potter. I think this is a good comparison and was going to have Damhan-Allaidh go one farther than gruagachan and be able to hide his soul among multiple objects. This gives the magi PCs something to work on without direct conflict and while still less powerful.
Based on the material available in Hedge Magic, RoP:I, and all the other books, how would someone put Damhan-Allaidh together?
First, take a Rune Wizards from Hedge Magic/Ancient magic... Then add the Grugach abilities and the Infernal abilities from 'Infernal'. Mix a few of the Mysteries in to flesh out his powers, and cap it all off with Parma stolen from Hermetics...
Of course our game is quite advanced, but he has been lurking around in our game (causing trouble) for about ninety years...
One thought is to go the Faerie approach. It's one of the greatest Hermetic stories ever told, passed down to new generations of apprentices. He's become the boogey-man of Hermetic Myth. The story is just being re-told over and over by the Fey.
I personally like the idea of Damhan recruiting a group of British Hedge-Wizards against the encroaching OoH and teaching them infernal powers to aid them in their battle against the fledgling Order. The ability to grant Infernal Blessings (especially MR) would significantly even the playing field between the two sides, the ability to boost Casting Totals with Sacrifices would be of great benefit to Folk Witches and Vitkir, and Chthonic Magic, in combination with Cailleach Magic, would go a long way toward helping the Gruagachan mitigate their Difficult Techniques...
For Damhan himself, I'd recommend: Giant Blood, Diablerie and Debauchery are excellent additions to the Favored Abililites ascribed to Damhan Duidas (and presumably, also Damhadh-Allaidh). One might also consider the possibility that Damhan and Damhadh were the same character and add the Personae virtue as well...
I like "the Faerie approach" idea, but it suggests that there is a likely end to the story. I also like the idea of mixing a little rune wizards (from early Saxon witches), gruchagan magic with giant blood and shapechanger from native scots, and infernalism.
I do not see Damhan-Allaidh as favoring debauchery based on his "spider" modus operandi. Spiders are patient, plotting, and either stalk or trap their prey using cunning and planning. Looking back at the histories, Damhan-Allaidh was not just powerful, he also organized different forces to oppose the Order, was able to counter most of their early attacks, and rarely relied on direct assualt. That type of cunning runs counter to the passionate/ecstatic method of debauchery, and suggests incantation as long as he can still use human sacrafice to gain vis.
Diablorie is good as it allows him to call forth demons. I also like Effusion since he appeared to be able to defeat hermtic magi in open conflict (at Carnac and against Ignis).