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).
From the character's perspective: You behold the soul bag, a leathery vessel pulsing with an eerie glow. Stitches weave a tale of agony, each knot binding the essence of the innocent within its macabre embrace.