Many way you can skin that cat. When I ran Tomb of Annihilation, the PCs traded certain items to one of the night hags that allowed her to craft an Ether Staff with 7 charges, which the hag could use to attack across the boundary between the Border Ethereal and the Material Planes. It was pretty frustrating for the players, but thankfully was limited use, and they thought to cast see invisibility and were able to negotiate with the hag.
