My party is about to face a coven of Night Hags. (Hags gain special abilities when in a coven, meaning three allied hags near each other.) It seems like the party should be able to focus on one of the hags and break the coven fairly easily. If one of the hags were to use her action to go ethereal, is that a valid tactic to prevent the coven from breaking, or would it break anyway by virtue of one of the hags technically not being on the same plane?
Relevant text on the rules for the Border Ethereal found in DMG. pg 48 under Border Ethereal:
The exceptions are certain magical effects
The Hags have a special magic effect that when they are within 30 feet of each other, they have access to certain spells. Since Etherealness is part of their actions and can be performed without expending spell slots, it's not unreasonable to assume they would have practiced this form of magic quite often to work together.
There doesn't appear to be anything preventing it. So long as the Hag's maintain the 30' prerequisite, they have access to the spells. The planes overlap, so all that matters is vicinity. This is why they would be considered a CR 7 as a coven (and especially if you have a Blastlock who can obliterate them on the Ethereal plane and force them apart anyways.)