The term "Ethereal Troll" really seems it should be a living creature.
The best question is what CR to start with.
A Troll is CR 5
A Phase Spider is CR 5
A Xill is CR 6
Maybe a Ethereal Troll should be a higher CR than the Phase Spider and Xill, and should be an Apex predator that hunts the spider and xill as primary source of food.
