When it gets down to brass tacks, I need a solid, RAW means of becoming a fiend. On top of that, I need a good way of doing it without it being overly broken mechanically. Looking for maximal RP impact, with minimal CO impact, if you catch my drift.
But when you deal with RP there is no certainty !
Nar Fiendbond is a spell from Lost Empires of Faerun that grants the half-fiend subtype to a humanoid. You still need to pay for it (i.e. you won't level for a while). You become an outsider ; you could use rituals from SS to grant you the Lawful and Evil subtypes but they're ridiculously expensive and may not help. Still, it's a first step towards fiendom.
Another ritual from SS uses Wish to turn you into another creature but again you'll need to pay for he LA and RHDs (by switching out your class levels I think).
Yes, RP is an uncertain environment, but there are "RAW" pathways to achieve fiendom as you just showed. The point is to find multiple mechanisms for achieving the goal and then seeing which ones are possible in the "RP environment", aka the campaign. Anyways, the idea of selling your soul to higher powers is dangerous for a reason. One day the DM could just up and say that someone has come to collect. If chaos ensues as a result of multiple claims, chances are it'd be over your head and your ability to affect the end result would be slim. In other words, the risk is high and the rewards uncertain. Just a bad idea all around. And you can bet that no devil will be laughing when you give them the sole of your shoe.
As a mortal how are you able to lay claim to a soul you've corrupted? The credit of the corruption goes to the devil initially involved and he has no obligations towards a mortal. In other words, you're not part of the system and anything you contribute to the system could be usurped by the devil who corrupted you in the first place, getting you no where, but on the mercy of said devil.
Night hags are known merchants of souls, and have no part in the infernal hierarchy. The last one that did ended up getting remodeled into a house (so to speak). So there is precedent for this sort of thing.
Precedent for bartering with souls, sure, but currency cannot always buy what you want. I was arguing that if you're not in the heirarchy of Hell, there's not necessarily a way to BUY a future promotion. I can talk to the DM and see what he thinks about this, but until I do, I file such schemes under the "not-so-certain" pile. If the DM approves the purchase of promotion futures, then stocking souls would be an attractive option.
Trap the Soul being a convenient mechanic for doing precisely that.
But if finance isn't your thing, you can always go back to the fundamentals and go the fallen angel route. You'll at least end up an erinyes, and Asmodeus used that route himself.
I can't imagine that becoming an angel -> fallen angel would be any easier than just going straight to devil. I might be wrong, though. Did you have a specific mechanic in mind?
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True Name magic brings a couple interesting options to the table.
1. Fiendbinder isn't an absolutely terrible PrC, though it is difficult to justify losing caster levels for the abilities it brings. RAW isn't completely solid, either, but "Archfiend's Favor" has to count for something in Hell.
2. Unname: Supposedly destroys the soul, true name, and body of the individual. Although there still exists some ways to bring the person back... not sure how solid this is, but perhaps one might use this to abrogate claims on one's soul by destroying said soul and assuming a different one, under another true name.