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.