he could be using the souls to hold back on the astral plane a powerful enemy who would bring trouble for him and the adventurers as well on the material plane. that could make room for an entirely different quest. i'll keep it
Have you read Waterdeep: Dragon Heist? One of the four villains you can choose from are a pair of nobles that sold their childrens' souls to Asmodeus and want to break the pact, but that requires them to sacrifice a bunch of money and 100 other souls to Asmodeus in exchange. You could do something like that.
Souls used as an energy source?
My lich pretty much uses souls as batteries. Granted, slowly consuming one soul was enough to get his phylactery going for weeks, but hey, you can change stuff so 100 souls are required to fuel whatever your evil wizard (perhaps a necromancer) wants to fuel?
How powerful is this wizard guy? He could be looking for a batch to fuel a new -very high level- spell he's developing. Or to have a "bank" to experiment with..."What happens/how does this spell function if I use 5 souls? Hm. Nothing. Ok, cast it again using 10. Not bad. <scribbles notes. adjusts calculations> What if I pour 42 into the casting? Oooo. I like that!"
It could offer the heroes the opportunity (or time sensitive motivation) to stop/get the big bad to FREE/release/save the accumulated souls before they are all "burned up." Drop some tip that the souls are for them to "use up"...and maybe there's a couple of friends/family members/loved ones soaked up in the soul harvest. So they need to get to the evil stronghold. Stop the uber-wizard. You can't bring them all back to life. BUT you can make sure they are freed to their just afterlives/peaceful rest/what have you.
Clearly, this needs to be a very high level (8th or 9th) spell. Almost certainly it should be in the Necromantic area. Maybe some kind of "mass enervation?" Or create a "Necrotic eruption" of the Negative Energy plane (or the setting's underworld) into the Material plane (because the evil wizard is looking for his Ophelia or whatever). Or a "mass-scale vampiric take-others'-lives-to-add-to-my-own" [or immortality/entering lichdom?]. Which, as suggested, could just be the point of taking the souls in the first place (extra year per soul sounds about right to me). Living an extra century -for a human wizard, in particular- would be a massive boon, making the indisputably evil dealing in soul-sucking "worth it."
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