In a lot of literature there is the one figure you go to in order to access Humanity-threatening power, i.e. basically the equivalent of the evil mastermind/tutor who helps you access your own diabolic power.

Would this best be presented by a powerful sorcerer with a stable of bounded demons (so you go about binding his demons from him, with his permission), a demon in himself, or is it better if he simply has a high Lore value for rolling up some bonus dice?

I'm leaning towards the third option, but I'd like some opinions on this. In play, I'd imagine it as the master talking over personal issues with the apprentice, encouraging him to seize his more base emotions; a series of master's Lore or Will vs. player's Will (with roleplaying bonuses of the character's breakdown adding to the master's roll), each roll accumulating bonus die for the next roll, until the player is ready to Contact/Summon/Bind.

It really depends on the setting variant your game is, doesn't it?