A Cthonic Realm?

Have an idea for a campaign inspired by the copy of Cthulhu Dark Ages gathering dust on my shelf.

What I want to do is run the Ars Magica setting with the inclusion of a new Realm: The Cthonic Realm and all the nasty alien beasties that are a part of The Mythos.

New realm though, means new aura complications and schticks associated with that realm. Also one of the themes of The Mythos is the corrupting influence of blasphemous knowledge. This is easily represented by a form of warping similar to that of Hyperborian blood except without the glowing and more tentacles.

Problem is, whats the incentive for learning Cthonic Magic? I was thinking making Cthonic Magic able to ignore Cthonic might, or even be able to bypass a minor limit of magic or two?

Any thoughts?

Summoning, banishment and otherworld travel would be their forte, I think. Some advantages related to that? Banishment that does not need to penetrate - only match might, perhaps?

This idea was discussed at some length here: