Forge Devil
Neglected to mention in that lost post that all of those fiends were demons. Now I’d like to introduce you to a devil. The primary difference between demons and devils (at least according to the D&D cosmology) is that demons are all about chaos and devils are more about stealing souls. So while demons can be unruly rage monsters, devils are more wily, tactful. To gain power they’ve got to have some people skills. Or at least have something to offer a gullible human that they want and will be willing to make trades or concessions for.
Which leads us to our friend the Forge Devil. Adventurers become stupid trustful of strangers in two situations. First, if you put bewbs on an NPC, then male gamers automatically start acting dumb. Second, they will split the party or slaughter each other over the promise of magic weapons.
Which is where I got the idea for our friend the forge devil. Most devils use weapons, as opposed to demons which mostly rely on natural weapons (except for the top-tier demons, Balrog Balor and Marilith). So somewhere in the Nine Hells, someone needs to be crafting devil gear. While most forge devils are content in practicing their craft, some may decide to become ambitious, and begin poking their noses into the realm of mortals, discovering just how few magic swords it takes to corrupt enough people that they can bargain for souls.