Rereading these again for a character concept I'm developing and it strikes me the mechanics behind these are based on Personal Powers (the text considers them equivalent to a Level 15 spell effect), balanced out so as to not require a Might expenditure and/or Fatigue level with 0 Penetration?
(see details in The Contested Isle, pages 101-104 for exact descriptions)
Looks like they're learnt as a Minor Virtue (effectively known to the Mystagogue given Ease Factor 15).
If so, they could effectively be used to create various Faerie or Magic entities (either as a Minor Virtue or a Minor Quality).
Looking at the text box on p.101, it says "The geas, which is also somehow related to the feat, is in effect a charm that controls the feat." So I suppose it's like a Faerie charmed virtue in some respects.
I can't remember the thinking behind the mechanics (that was Mr Shirley) but I know they're awesome. I had a warband use them in my last session and they certainly gave the magi and their grogs a hard time.
Of course, Ball of Abysmal Flame is a great leveller, so the fight wrapped up fairly soon after that was deployed.
That's basically right. However, they are meant to be more than just the exercise of magic; they need to be intensively trained to learn and keep them, so they do have a physical component. A faerie would just use a power and make it look like he was so skilled, but a careful observer could tell the difference. A magical being might go the more direct route rather than undergo initiation, so whereas the results would be the same, the manifestation would be different.