As far as I know there is possibility to loot Bag You Desire from monsters known from the Soul War Quest area if we've got at least 1 active taint.
... and here is my doubt:
Do all of monsters have a chance to drop this bag? Or maybe only the monsters which reacts to our taint have this possibility?
For example when we're doing mission in a spooky version of Thais only Many Faces have the possibility to teleport next to us, so only this particular monster reacts to the taints.