The PCs, having traveled to the Ethereal Plane, will encounter a monster that's made its home there for some time and that casts spells as a wizard. I'm considering having the monster prepare the 2nd-level Sor/Wiz spell ethereal chamber [evoc] (Spell Compendium 84). The spell has the entry Target: One ethereal creature and the spell's description says, in part, that
You create a translucent prison of pure crystalline force around a target on the Ethereal Plane. You must be able to see or otherwise target the creature you wish to affect. The subject affected by the chamber cannot move on either the Ethereal Plane or its coterminous planes for the duration of the spell but is otherwise unaffected. The subject cannot move onto another plane while inside the chamber, including the Material Plane.
The subject within the crystalline prison cannot make physical attacks or be attacked and is unaffected by most spells and supernatural abilities.…
The spell seems to presume the caster is on the Material Plane and the target on the Ethereal Plane, but if both caster and target are on the Ethereal Plane is the target still considered ethereal enough so as to be potentially affected by the spell, turning the spell ethereal chamber into a low-rent version of the 4th-level Sor/Wiz spell Otiluke's resilient sphere [evoc] (PH 258)?