The infernal book is great, but I was disappointed in how little there was for Hermetic Magi to vet from infernal meddling. As written I did not feel like there was much temptation toward imperialism. The following ideas are half baked but are the start of my thoughts toward a more tempting infernal power.
Goals: Retain supremacy of hermetic magic, while making infernal temptation really tempting.
Would grant the following as Infernal Abilities to selected Demons:
Grant Infernal Art Affinitiy: (By Form, 2 pts), The demon grants the target a False Affinity to a Form Art, immediately recalculate the previous experience and use that new value for that Art. The art is now Corrupted and appears infernal to magic senses/intellectual, and has some aspects that appear more dark. The demon is now an Arcane Connection to the magus. The power has duration: Host, (or whatever that its called, sorry no books on me) and lasts till the Magi tales communion, confession or other sacrament. Demons can only boost forms that match their own form.
Grant Infernal Puienssance of Form (by Form, 1pt) Simular to the above but only grants a plus three bonus.
Grant Infernal Abilities (Corpus or Mentem, 1 or 2 pts): similar powers as above for Abilities including supernatural abilities. A given on would need a power for each ability.
Grant Infernal Alignment (Vim, Varies 1-5) The demon may Grant the magus a temporary alignment to the Infernal Realm, she may use the Infernal Realm Aura bonuses up to 1/2 The value, as if it was a fairy aura. Same duration as above. The demon is similarly aligned to the Magic Realm. Cost equal to the maximum aura bonus that can be used.
False Mastery (B Form, 1-3 points) Grants 1 to 3 Matery abilities for a spell that matches the form of the demon. Same limitations s above.mastery abilities are chosen by magus player.
The Power of Vis (Vim, 2 pts) The Magus may add +3 when using infernal vis for a spell . Far vis causes a -1 penalty and magic vis only offers a +1 bonus.
These powers will produce demons that Magi will want to deal with and will corrupt the order. Obviously they ave to penetrate which requires trust.... which will be interesting. Presidents costs the demon extracts for these powers should be in proportion. Also since they can be canceled by the demon or the Magus' s actions they should be ongoing costs. Taking covenant during those demons should be a significant challenge.
On another note, I think that far and infernal vis should be more prevalent, offering a +1 bonus each or normal bonus with additional botch dice. College ting infernal vis should be morally dangerous. Likewise, using this vis in other auras (magic s in far aura, infernal vis in magic aura etc) should do something... perhaps something the Criamon mental demons. That trouble I'm aura at a later time. So that covenants that are using Infernal Vis or Fairy vis should have strange happenings. Or is using it to push through. the Divine will leave behind weak demons and fae things (and magic things) that will cause problems them to deal with I'm a year or so.