Ethereal is demon-like, nightmarish creature appearing for the first time in the Hearts of Stone expansion. For Iris, it takes on the form of her husband, Olgierd.
The human mind is as wild and unexplored a place as any land far beyond the seas. It is inhabited by manifold dangers – fears, painful memories, traumas. These lurk on the edge of our consciousness, like predators waiting for a moment of weakness in which to reveal themselves... and destroy us. – Arturius Vigo, "The Essence of Spells and Illusions."
The painted world created in the mind of Iris von Everec was mainly populated by figures from her memories, but was also home to dangerous, predatory creatures: the embodiments of her fears. The most dangerous of these, the Ethereal, invoked her greatest fear – and wore the face of her husband.
The Ethereal resembled Olgierd in appearance and also fought using a technique similar to his. A group of six such creatures lay in wait for Geralt, yet at first only one engaged him in battle. This one's death awoke the next, which was more powerful than the first, and so it went to the end, with a stronger ethereal replacing a fallen weaker brother until all were defeated. Yet Geralt had to be careful, for the slightest amount of damage dealt to a being awaiting his turn would cause him to come alive and join his still-living comrade in the fight.
In the end, the best tactic against these nightmarish beings proved to be a well-timed counterattack.