A new player is creating an Ex Miscellanea magus of the Damhadh-Duisan tradition and is interested in taking Incantation as their major non-hermetic flaw. When reading incantation in RoP:Infernal it is always used in conjunction with one of the six unholy powers, but no other infernal virtues are listed for the tradition in the Ex Miscellanea section of HoH:Societates. Consumption and Malediction would seem most appropriate from the discriptions of Damhadh-Duisan's powers.
How would you approach this issue? Create new guidelines for Incantation with Hermetic Arts? Substitute certain Hermetic arts for certain infernal powers (Consumption with Corpus? Malediction with Perdo?) Hand wave an infernal power or two into the tradition?
There are already rules for using them with Hermetic Arts, under Chthonic Magic. I would give the tradition four Favored Abilities: Incantation, Chthonic Magic, at least one Power, and one other. If the player wants to start with more than one of those, they pay for the Virtue to get it at the start. Otherwise they're more limited when starting, but they can pick up the other stuff later.
Do the favored abilities you suggest advance as abilities or arts? If they advance as abilities this looks like not something a magus character will invest in very much. The Incantation/Consumption spell Break the Body, Bend the Will / One Can Injure, One Can Kill, is level 30, to heal a light wound (great name, btw). Even dropping this to a touch range spell would be level 20 (and with both malefica advancing as abilities, that's around 300 xp to get within a 50% chance of casting it, assuming no auras).
This might work for diabolists with nowhere else to put their experience into, but a Hermetic magus? They just have far more power at their fingertips for less effort going Bonisagus' route. Hell needs to catch up if it is going to tempt the Order.
IIRC you can design hermetic spells/rituals using effects guidelines from the infernal method and powers. You use the normal combination of TeFo but substitute Philosophae and Artes Liberales for the two infernal abilities.
I might be wrong about this but I know that you can do the same with faerie and divine method and powers.
They advance normally. Being Favored doesn't change how they function at all once you've acquired them. It only means you suffer no penalty at all from other Arts or Supernatural Abilities when you first learn them.
Yes, that's the general problem with all non-Hermetic Arts and Supernatural Abilities for Hermetic magic. Generally they just draw away experience and don't give much value. There are certainly exceptions. Those Methods/Powers add very little to Hermetic magic, so yes, generally it's just a big waste of experience. Even Chthonic Magic, which does do a lot of Hermetic magic, gives most of its benefits at a score of 1; so it's still an experience sink. The Goetic Arts combine better with Hermetic magic than the Methods/Powers as they allow ReVi to do much more. But they're still experience sinks since they give nearly all their benefits at a score of 1.