This may have been answered already but what exactly does the night hag soul bag contribute to? In combat it's useless. As a PC if they take her soul bag with an evil soul in it, is it with anything to anyone really? Maybe able to use in a ritual? Just curious, running a one shot tomorrow using an adventure that I got off DMG and it says after the PCs kill the night hag part of the loot is the soul bag.. Just wondering what if anything it would be used for or is the really a DM's choice?

Thanks

maybe she's collecting the souls not for herself, but for a Devil that is her superior?
perhaps that soul in the bag is very important to that devil. resulting in the Hag to bring in an entire coven to try and get it back. a recurring villain.
perhaps it'll even lead to the devil itself sending more forces after the player to retrieve the contents of the bag.
maybe the players will make their own bargain with that hag coven or devil for a nice plot twist.

The best thing I could think of is Soul Cage...

A spell from Xanathar's Guide To Everything

Necromancy

Level: 6
Casting time: Special
Range: 60 feet
Components: V, S, M (a tiny silver cage worth 100 gp)
Duration: 8 hours

1 reaction, which you take when a humanoid you can see within 60 feet of you dies

This spell snatches the soul of a humanoid as it dies and traps it inside the tiny cage you use for the material component. A stolen soul remains inside the cage until the spell ends or until you destroy the cage, which ends the spell. While you have a soul inside the cage, you can exploit it in any of the ways described below. You can use a trapped soul up to six times. Once you exploit a soul for the sixth time, it is released, and the spell ends. While a soul is trapped, the dead humanoid it came from can’t be revived.
Steal Life. You can use a bonus action to drain vigor from the soul and regain 2d8 hit points.
Query Soul. You ask the soul a question (no action required) and receive a brief telepathic answer, which you can understand regardless of the language used. The soul knows only what it knew in life, but it must answer you truthfully and to the best of its ability. The answer is no more than a sentence or two and might be cryptic.
Borrow Experience. You can use a bonus action to bolster yourself with the soul’s life experience, making your next attack roll, ability check, or saving throw with advantage. If you don’t use this benefit before the start of your next turn, it is lost.
Eyes of the Dead. You can use an action to name a place the humanoid saw in life, which creates an invisible sensor somewhere in that place if it is on the plane of existence you’re currently on. The sensor remains for as long as you concentrate, up to 10 minutes (as if you were concentrating on a spell). You receive visual and auditory information from the sensor as if you were in its space using your senses.
A creature that can see the sensor (such as one using see invisibility or truesight) sees a translucent image of the tormented humanoid whose soul you caged.

This is pretty over-powered in my opinion.  You've taken an item from a cr5 mob and granted a treasure that effectively allows daily casts of a level 6 spell.  Normally a player would have to be level 11 to cast that spell.  I'd nerf it it a bit.  Maybe only one of the effects per day....

Well a soul bag would be a holding spot for a Night Hag to change a soul into a Larva, probably by transporting them to Hades/The Three Glooms/etc.  Then they are used per the DMG:

"Hades is crawling with larvae. Night hags, liches, and rakshasas harvest them for use in vile rituals. Other fiends like to feed on them."

Source: Hades and Larva

They also have been used as currency among fiends in prior editions and settings.  So you could use that as well. 

GLHF

I would think figuring out what to do with the soul would make a nice side adventure.

Find a way to identify or communicate with the soul

Discover it's the soul of a pirate famous for having hidden a huge treasure that was never found and try to convince the soul to lead you to it

Discover it's the soul of an evil ruler who was killed by the current evil ruler, and they will give you lots of good advice on how to kill the current ruler and get revenge for them

Go on a quest to learn an ancient ritual that will let you bind the evil soul into a homonculous or robot, where it can atone for its evilness (or continue it) by serving you

Give it the power of speech and the player has a wisecracking evil soul sidekick

Find a devil to sell it to

Unless dear Xabbu... it comes at a terrible cost...

I can imagine the devil/fiend a night hag perhaps served under wondering where its source of souls are and using scry and locate object  to trace where that soul bag. Could end pretty badly for anyone who thinks they can get such power at no extra cost. And you could put a price on trying to use it. Or restrict the use of it to specific classes.

EDIT: This is going off the idea of using it as a magic item that has a daily Soul Cage  charge.

Quote from Xabbu >>

This is pretty over-powered in my opinion.  You've taken an item from a cr5 mob and granted a treasure that effectively allows daily casts of a level 6 spell.  Normally a player would have to be level 11 to cast that spell.  I'd nerf it it a bit.  Maybe only one of the effects per day....

 The hag can only harvest souls with her soul bag when she kills an evil creature with her nightmare haunting. Assuming that the soul bag grants the hag a retextured soul cage, means that the player would be unable to cast soul cage, but benefit from a soul that has already been trapped in this way. Perhaps with a spell like dream or etherealness (which may be unnecessary since the player may already have access to soul cage) a way to use the bag to do the nightmare haunting, allowing for soul cage to be cast in a weird way. Another way the bag might be used in this way is in tandem with the heart stone if you allow players to use it to go ethereal once per long rest or something like that. Maybe the heart stone could be the thing that allows the nightmare haunting and the etherealness, allowing for an early albeit weakened soul cage which would be OP combo. Maybe too op...

Quote from alucardwoa >>

This may have been answered already but what exactly does the night hag soul bag contribute to? In combat it's useless. As a PC if they take her soul bag with an evil soul in it, is it with anything to anyone really? Maybe able to use in a ritual? Just curious, running a one shot tomorrow using an adventure that I got off DMG and it says after the PCs kill the night hag part of the loot is the soul bag.. Just wondering what if anything it would be used for or is the really a DM's choice?

 

Thanks

 If the hag is holding a player or npc soul that the players want to release for a resurrection then obtaining or destroying the bag is the goal of the encounter. 

There are also some mechanics in decent into avernus for trading souls to devils so that is another use.

 On the more dm fiat/homebrew side  there is also treating it as soul cage or an ingredient for making a magic item, a trapped soul would make a good ingredient for making a sentient magic item. In the xanathars rules a single night hag is cr 7 so is in the range for an uncommon item but a defeatign a whole coven could be worthy of makign a fairly powerful item. 

I have enjoyed the lore of the Fey for quite awhile and I actually like what the creators did with the descriptions of the night hags tools. They didn't give too much detail as to trap story tellers into certain mechanics. They left it open for creativity. The bag is just a container specially made to work only for the night hag. 

It is fascinating some story tellers chose to interpret the need of the soul as a gift or payment to a higher deity. Then some story tellers choose to make the soul a ritual component the hag needs for their own self interest. Either way makes for a very flavorful story and makes for a great game.

But in my opinion that bag has no magic ability past being a prison for a soul that is trapped by the magics performed by the night hag. It is as useless to anyone else save the person who is motivated to retrieve the soul inside for (name any purpose). This is a great open invitation for any story teller to craft to their tale! 

The bag is used by the night hag to kill people.  See the ability Nightmare Haunting that the Hag has.

It is also intended to be a DM plot point. You must stop this evil person before she not only kills the victim, but steals the soul, preventing them from being resurrected.  Then, after she has succeeded in killing someone and adding the soul to the bag, you have to kill the hag to get the bag.

Perhaps you cast speak with dead on the bag or break it, freeing the soul and then ressurect someone.

As a DM, I would also rule that only hags that have a soul bag can be powerful enough to have Lair Actions or Regional Effects.  Get rid of the bag, those things vanish.

@Mog_Dracov - That might work kind of, straight from the descriptions based in the monster manual but really is missing a lot of lore and legend. There have been multiple opinions given in Youtube on the lore and legends of the hags if you want to add that flavor.

My understanding of the Hag from the various offerings of TSR and like sources is they are the Merchants of the underworld and their primary sale item is soul larvae. The bag helps them create specific soul larvae, but in essence soul larvae are just evil souls that have passed from the mortal world and end up in Hades which is home to the Night Hags. It is up to the story teller, but a handful of hags collect them on Earth for special purposes or sales to demon and devils. 

By that I have concluded the bag is just storage only. It is made from soul larvae and are labor intensive, so hags hold onto them so they can sell the larvae instead of using them to store more. Your ideas of identifying or releasing the soul larvae is left to the story teller, but realize they can only contain evil souls. 

Now as a DM or Story Teller you can make anything up as you go. So I always encourage to have fun. I would suggest Looking up Soul Larvae in Forgotten Realms, and possibly the  Demonomicon which I think is 4E.