This is just a quick, short little note today. I am looking at an encounter with my party involving ghosts. Ghosts are a bit special, in that they can hang out on the edge of the Ethereal Plane, in the Border Ethereal, where they can see into the Material Plane and can be seen from the Material Plane, but can’t be affected from the Material Plane (with a tiny handful of exceptions). Most creatures in the Material Plane can’t see creatures in the Ethereal Plane (even in the Border Ethereal) without some special trait or magical effect, like True Seeing. Except with ghosts, and only one other exception which I could find (phantom warriors, which appears to be merely a type of ghost). I was wondering if a cleric’s Turn Undead would affect a ghost on the Ethereal Plane, as well as all the Undead on the Material Plane close at hand. While I was looking that up, I discovered that many other people over the years were similarly confused about this, and most/all of those people did not get a final resolution, past randomly guessing. I believe that I have unearthed a final resolution, but it was through a fairly roundabout journey.
So here’s the deal: most effects on the Material Plane will not affect the Ethereal Plane (and vice versa), with few (but there are a few) exceptions (e.g. things made of magical force will be found on both planes, according to the Dungeon Master’s Guide p.48). Turn Undead states that “Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw.” The ghost can see you (and you can see the ghost), so the question is whether the ghost is within 30 feet of you. The ghost may appear, visually, to be within 30 feet, but it’s on another plane, so what does that mean, in relation to the spell’s requirements? Well, according to Jeremy Crawford, lead rules designer of Dungeons and Dragons 5th edition, the Ethereal Plane and the Material Plane are infinitely far away from each other. As a result, casting spells which require any degree of proximity will fail between the two planes. You heard it here first, people.
Now, does that mean that you can’t affect a ghost while it is in the Border Ethereal and you are in the Material Plane, even though you can see it right there in front of you, taunting you? That depends on what you want to do with/to it. If you want to keep it away from your party, you can cast Leomund’s Tiny Hut, and as long as the ghost wasn’t within the hut’s radius when you cast it, the ghost won’t be able to enter it in either the Material Plane of the Ethereal Plane, since it is made of force. If you are a MUCH higher level magic user, you could also cast Forcecage on it: this would either trap it either inside or outside of the cage for the duration of the spell (an hour). Honestly, since the spell costs 1500 gp though, it hardly seems worth it, since a spellcaster of high enough level to cast Forcecage should be able to deal with a ghost with very little difficulty, and 1500 gp worth of ruby dust is nothing to sneeze at.