Can a Night Hag catch souls without Nightmare Haunting?

Question

Every night hag possesses a Soul Bag which is described as follows (cf. MM, p.178).

Soul Bag: When an evil humanoid dies as a result of a night hag's Nightmare Haunting, the hag catches the soul in this black sack made of stitched flesh. A soul bag can hold only one evil soul at a time, and only the night hag who crafted the bag can catch a soul with it. Crafting a soul bag takes 7 days and a humanoid sacrifice (whose flesh is used to make the bag).

I was wondering if a night hag can capture evil souls only through their Nightmare Haunting or if they know how to do so when any evil character dies in front of them for whatever reason.

Answer 1

Technically, souls can be captured only using Nightmare Haunting

This answer is a result of a literal reading of the description of the soul bag:

When an evil humanoid dies as a result of a night hag's Nightmare Haunting...

If anything else was intended, it would say so, but it doesn't.

On the other hand, a DM can always adjust details as necessary. And this description is part of the lore rather than the basic statistics of the creature, which is easier to overrule without that much consequence. So if you are the DM, and require the hag to be able to capture souls some other way for the purposes of plot or world-building, then I would say: go for it!