How can the agent of a devil “capture” a soul in D&D?

Devils always want more souls for the Blood War. Mostly, they get those from lawful evil mortals. But as various lore – and this question What groups of souls go to the Nine Hells according to lore? – make clear, pacts with devils and even magical accidents can also cause a soul to go to the hells.

In my game, one of the party (a Warlock) owes a favour to a devil (their patron). I’ve decided that this favour will be to "claim" the souls of two NPCs in the town where they’ve just arrived. The devil in question is Dispater and the town happens to be home to two master blacksmiths, who Dispater would love working in his armouries for all eternity.

Presumably, the most effective way for our Warlock to go about this – if they choose to accept the task – is to try and tempt them with some kind of Faustian bargain so they will make a pact of their own. However, I’m bound to be asked whether there’s any other method. Is there?