As a high-level cleric, I would like to use the spell etherealness and build a hidden temple of the gods on the ethereal plane at the same spot where I have a normal house in the material plane.
I cannot find any good information on what happens if I attempt that.
Will the temple drift away in the ethereal plane (can some funny NPC traveler push it away), or is it a fixed position? Will the spell fail because there is no real firm ground? I need to have a better understanding for how this works before I go to my DM.
Best Answer
This is perfectly possible to do...but there is a gotcha
D&D is an exceptions based game.
(PHB page 7)
You need to have a rule to justify something exceptional. A place where people can walk and travel (explicitly) not having ground is exceptional.
The general rule is that the Border Ethereal is a mirror of the relevant plane:
(DMG page 48)
So, what is a location?
Oxford Dictionary defines location as
Next we ask what is a place?
Oxford Dictionary defines a place as:
Thus in order for there to be a "corresponding location" there has to be ground for that location as the originating plane has ground (the Material Plane, Feywild, Shadowfell, Elemental Planes all have ground).
Finally nothing in the DMG description of the Ethereal Plane, or Border Ethereal, states that there is no ground on the Ethereal Plane. As there is no specific rule removing ground from the Ethereal Plane, it has ground.
This means the first line of the spell
is satisfied.
The potential "gotcha" of the spell for your plan is this line (emphasis mine)
As a result, unless you make the temple appear around you, you won't be able to enter your new Ethereal Temple while you are on the Ethereal Plane. The only way to enter your temple will be to go back to the bordering plane, go to a spot inside the temple and either cast "Etherealness" again, or "Plane Shift".
Aside: Dale M's answer relies on an interpretation of this line from the DMG:
(DMG page 48)
that the ground is an Object.
The DMG defines an Object as:
(DMG page 246)
Thus the ground is not an object for the purposes of the rules.