I think you need some homebrew mechanics to go with that homebrew lore to make it work.
Quite likely.
@Chaosmancer Oh gosh, several ways you could go narratively. Why would a wizard select a fiendish familiar whose ultimate aim is to drag his soul to the Abyss?
- Maybe the wizard doesn't know/isn't aware of his familiar's true purpose.
- Maybe the wizard doesn't care about his soul, and is only concerned with here and now.
- Maybe the wizard views it as an acceptable price, a la "better to rule in hell than serve in heaven", or a necessary price given the challenges he faces, a la Constantine.
- Maybe the wizard thinks he's more clever than the measly familiar and will outsmart it, gaining all the benefit with none of the cost.
- Maybe the wizard believes it's a case of "better my devil, than it serving someone else who's truly wicked."
A lot of these kind of paint the wizard as... stupid.
A) Not aware of how magic works or how dangerous fiends are? I know fiends technically fall under the purview of religion instead of arcana, but not knowing that in summoning a fiend you are risking dangerous forces seems more ignorant than I would expect. Especially since Find Familiar is a 1st level spell, this is basic magic we are talking about , so I would expect wizards to learn the nuances of it rather quickly.
B) Two issues. One is that "I want a familiar who is sloppy and dangerous, but will act quickly" is again just poor decision making. Two, even if they do, they can dismiss the familiar and summon a celestial familiar later. Which raises all sorts of questions about how much sway the demon familiar will have over their soul, and also makes one wonder why a wizard would keep it around after an accident or two.
2B) I want to side bar here to point out, that familiar stats are generally thought to come from the animals you summon. Meaning all of them have sub-human scores. The Frog would have an intelligence of 1 and a Charisma of 3, leading to hefty penalties on things like tricking their master or convincing their master to take a certain course of action.
3) Sure, works for some wizards. Except, the benefit they are potentially losing their soul for is "works faster but is sloppy." You'd have to be stupid to take what is overall a neutral aspect and add to it the potential for the servant to betray you and drag you to everlasting torment. I mean, imagine you are working in the food industry. Do you keep the employee who is potentially stealing from the register because he works fast, even though that means he sometimes messes up orders or breaks plates, costing you even more money. OR do you take the employee who works a little slower, is completely trustworthy, and always does a good job? This isn't a choice. You always want the second employ. They cost you less and are a better employee.
4) This can work, especially since as mentioned, most familiars are stupid and uncharismatic. I could see incredibly prideful wizards taking on fiendish familiars just to prove that they can handle it and that even with a poor familiar than can do better than their fellows.
5) This makes no sense. Better to summon a devil to serve than to summon a celestial who will work with me, because then I won't be serving a really evil master? That makes zero sense unless the wizard is just resigned to having their soul taken to the Abyss no matter what they do in their life. Which, could be a fun plot for a single character, one who is marked for the Abyss and figures raising their eventual master rather than risking a roll of the dice is worth it, but that is a hyper-specific story.
And again, I think the pattern emerging here is "sloppy but faster work" when the familiar does so little is a bad exchange. You aren't really gaining anything except your orders being accomplished moderately faster, so you might be saving a handful of seconds at the risk of your soul? That makes no sense.