As in the title, can a PC take another mortal (NPC's) soul? I am playing a tiefling in Descent into Avernus and have met a particularly evil character. Most of the info out there talks about how a soul bargain works with a devil (or demon). I have looked into Mordenkeinan's which has lots of info about Devil progression but I can't find any definitive info that says whether PC's can do it.
Can my PC leverage another character (an NPC) into giving his soul?
Your DM is in charge of the world, even for prewritten adventures in official settings. There are no character features that mention anything close to what you want to do, so only your DM can answer this for you. There might be official lore on similar situations but your DM is not forced to use any prewritten lore, which is why I would slightly discourage searching through books meant for DMs like Mordenkainen's as a player wothout consulting your DM.
Baldur's Gate: Descent into Avernus (p. 212) has a section on how diabolical deals work:
These deals are more than mere agreements; they are cosmically binding exchanges in which a devil grants a mortal character some measure of power for a price. Once an agreement is reached, the deal is sealed with a binding contract, which both parties must sign.
This is not a normal negotiation. It is something special granted to devils by the multiverse. The way this is written, only a devil can offer such a deal. That the deal is always with a devil is reiterated repeatedly in the section. And not even every devil can do this:
Least devils can't enter into deals. Lesser and greater devils can enter into deals either for themselves or as an intermediary for their infernal superiors. Archdevils are the only devils able to freely deal with creatures outside the Nine Hells, usually when mortals invoke them to beg their favor.
A Tiefling is not even a Least devil when it comes to its station. It's a humanoid, not a fiend (devil), even if it has infernal blood in its lineage. So, under the rules, as a Tiefling you cannot enter into a bargain with another mortal and have it enforced by the laws of the multiverse, as you are no devil. There is also no character class that has a feature that would allow you to enter into cosmically binding deals with another mortal for its soul, not even in the warlock class.
What you and your DM agree on, overriding the written rules for the sake of a story, is of course always up to you and your table.
