An ethereal unit has a 75% chance to negate any non-magical damage inflicted on it. This includes:

• Most weapon damage
• The 1 point of Damage from being even partially repelled
• The 1 point of Damage from avoiding a trample (as well as the full Damage of being trampled)
• The Poison gained by inhaling nonmagical Poison Clouds

Ethereal beings can also scale walls, both in battle and outside them, and may not be frozen as a result of suffering cold damage. They also suffer no consequences from bleeding or burning; sure, their arteries might be opened and they might be going up in smoke, but that doesn't matter when they're immaterial already. Last but not least, they're immune to most forms of Entrapment, with the exception of False Fetters and Bonds of Fire.

Etherealness does not protect against Internal Damage, such as the 4% conversion rate of Fatigue past 200 into HP damage, or the damage done to one's body by Disease.

To know if a damage source is magical, check for a white star left of its damage number. You should also check for this on spells; several combat spells have nonmagical effects, most of them being pure Earth ones such as Blade Wind or Earthquake. Interestingly, spell effects that target bones to deal damage or Afflictions are considered non-magical for this purpose.

While commonly associated with Ghosts, Etherealness is mainly an Astral buff;

• The Etherealness Bless provides it to Sacred units, but requires 10 and 3 on your Pretender. (It's the most expensive effect!)
• The Body Ethereal spell provides it to anyone, with a range of 5 squares and an AOE of 1. It's a Level 4 Alteration requiring 1.
• For 10, you can forge a Robe of Shadows at Construction 4 to give Etherealness to your Commanders. Death offers the Wraith Crown, providing Etherealness in the Head slot instead of the Armor slot, but it comes at a later Research level and requires 40.