I have an idea for a low-level PS adventure, in which good/neutral characters end up having the choice to work for one of the Lords of the Nine or some other evil Power directly or allow the Multiverse to possibly suffer some kind of major shakeup.
The basic idea is this: Someone has something the fiend wants, but can't get, because they're in Sigil and hiding out. Of course, the item is such that if another fiend gets it, it could have serious repercussions and allow the balance of power of the Lower Planes to tip. Thus, there's only one option left: get someone who's relatively unknown to the Sigilian and Planar community at large to get it for him.
The advantage is that if the PCs actually manage to retrieve the item and survive, they are now likely to be quite rich, even by adventurer standards... and can even perhaps bargain for a favor from the fiend (however, this is rather like a wish from a genie... the chances of things going wrong in some fashion are almost 100%) which would effectively 'make' them as adventurers to be reckoned with.
The downside is that whatever happens, they'll likely to have to be on constant guard, since some will want to know what the item is so they can get it for themselves, others will simply want to kill the PCs, and yet others will now try and use them as pawns in Lower Planar politics. Since they're still relatively low-level, there's not necessarily much they can do about all of this.
If they don't get the item, there's many of the downsides listed above, but the PCs also have to contend with the worry of maybe having started an interplanar war, or simply escalating the Blood War further. However, they're also somewhat less likely to get kidnapped, assassinated or suffer other unsavory consequence.
In the end, it's rather like the phrase "damned if you do, damned if you don't" in a slightly more literal fashion. I was assuming that, for the sake of Paladins (and the believability of the storyline) that the fiend would likely travel to some fiend-controlled area on a Prime somewhere, disguise themselves as an ordinary mortal, and bargain with the PCs in that guise. Any thoughts on that idea?
Sounds good; my only suggestion would be that if the characters are low level they would never be interacting in anyway with a cosmic entity as grand as a Lord of the Nine or a Power. If you want the characters to be low level have them work for something around the lines of a Cornugon or Pit Fiend at the most.