Pbartender said:
Likewise, you could feasibly use the familiar as the wizard's "spellbook"...
Yeah, I'd think the right way to do this bit would be to have the familiar replace the book completely. After all, it'd be the source of
all the caster's magical knowledge. And, obviously, it wouldn't
really just be an animal somehow given intelligence by a 1st-level character. Instead, it would be some kind of Outsider either inhabiting an animal or masquerading as one.
This kind of bit might work best in a setting where magic is (rightfully or wrongfully) suppressed by the authorities, making it impossible (or at least impractical) to establish formal magical schools where mortal spellcasters could share their knowledge, and leaving familiars as their only opportunity to learn. (Which, of course, suggests that whatever's behind the familiars might also be behind the suppression of magic. For fear of such sinister influences, the authorities would be creating exactly the situation that lets them flourish.)
But what do the familiars get out of the deal? Old witchcraft lore often has them suckling their familiars on their own blood (or other fluids). Maybe it's less about selling your soul in one go than trading off some kind of life essence at a rate that keeps the supply renewable (bump the caster's HD up to the next die, but require them to pay 1 HP per level every night to keep the familiar happy).
Of course, it'd be a lot more interesting if they asked for other favors from time to time. Maybe things to keep the image of magic the way they want it. Maybe things to help out their other mortal allies and servants. Maybe weird, nonsensical acts that seem inconsequential or absurd to any mortal, but are exremely important to events on some other plane.
I think the most fun way to play these familiar spirits would be if they were divided into two or more rival factions, pitting their servants against each other in an endless--and possibly pointless--secret war. Also, while
sinister is definitely a good way to play them, it might be best if no faction could be safely classed as good or evil. They aren't out to hurt people
per se, but they'll certainly manipulate the hell out of everyone, and if someone does get hurt, that's not really their concern.
Damn, now I'm really thinking about this.