Soul

A small, concentrated deposit of ether contained in every humanoid living being. This is what distinguishes humanoids from other creatures, whose bodies contain only trace amounts of the unrefined life element.

The ether in a creature's soul differs in makeup from the ether used in offensive energy blasts, barriers, auras and the like. This can be inferred from its more yellowish green colour. It's a special configuration of the element, and requires much more advanced affinity and skill in order to be similarly manipulated. It can however still be perceived without sensory input by ether elementals in the same way as other life energy. This is one of the advantages ether users have over other elementals: being able to perceive nearby life forms at all times.

A sufficiently powerful ether elemental is capable of accessing any nearby creature's soul ether in order to do any of the following: control their mind to varying degrees depending on ability, either invigorate or drain them physically and mentally, or even snuff out their life with a thought.

The soul is thought to reside inside the head, and there supposedly exist gnomish runes that can be applied to headgear in order to protect one's soul from such attacks.

While the soul appears to be what grants humanoids intelligence, there are reports of intelligent creatures without a soul, such as sphinxes or Surtur (as reportedly the ether in his soul burned away due to the excessive fire energy present inside him).

Hypothetical

When a mortal dies, its soul exists its body from the mouth, in the form of a glowing green orb of light. The fact that this does not happen to the six-legged beasts is part of the rationale behind killing them for food.