What can you use Souls for?

[Vaz]

Selling your soul is bad, right. But what can be done with it?

I've been reading through the Fiend of Corrpution, and I'm intrigued into making a build for it, as my Ardent that I'm running can qualify for it next level, and then take it the next time after that it levels.

I plan on eventually stealing the souls of some of my party members, and I'm intrigued into what can be done; as opposed to just using it to fund crafting.

So, what uses do Souls have; either as the end user, or for the "sellee"?

[Ionbound]

For most fiends, I seem to remember souls being used as fuel to increase in rank. Unfortunately, I can't remember where I read this, but I'm pretty sure it's the case.

[Grinner]

According to a D&D creation myth, the Devils built the Nine Hells from souls deemed undesirable by the forces of Good.

I can't think of any first party rules on the subject, though. If you work with the GM, I suppose you could get them converted to underlings or trade them to fiends.