Dear Larian,
"make a deal with a devil" - that sounds too Christian, or maybe I'm missing something about the BG lore? Anyway to me it sounds like something only a religious person (cleric/paladin?) would say and only in a Christian setting.
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This is actually pretty lore accurate. As Nyanko states Devils are the primary inhabitants of the Nine Hells, Demons are the rulers of the Abyss. The primary thing that differentiates them (beyond the unending hatred and ongoing millennia long war between them) is that Devils are Lawful Evil whilst Demons are Chaotic Evil (The original Demons are, IIRC, fragments of the original Elemental Chaos that existed before the current Planar Structure - whichever one the game is using).
A stereotypical Devil is pretty much the incarnation of all the bad lawyer jokes you have ever heard, they are ready, willing and able to strike a deal for your soul (or whatever else you are willing to offer - service, power, material wealth etc), but you need to read the fine print, a Devil will always strike a deal, but you can be sure that they will be the one coming out on top.
Although a Cleric or Paladin would probably know the details of a particular Devil or Demon, most inhabitants of Aber Toril would know the general gist (that Devils are always willing to bargain and that although you may get what you ask for, it probably won't be what you need and that the price will ultimately be more than you can afford).
