Etherelium magic is, fittingly named, ethereal magic which surrounds metaphysical aspects of the world, such as:
Time
Life and Death (Typically either resurrection or accelerated decay)
Identity (Usually combined with either Deceptive or Dark magic)
Human emotion
Human association
This magic is controversial not because of the contents, but instead because of the concept which surrounds it. Many people fear what a person with powerful magical energy could do if they're in magical control of abstract concepts, and the limits seem to all be endless, though one thing which people studying this magical energy have found is that it works best when paired with Deceptive magic.
Many abstract concepts of metaphysics revolve around human function and how they themselves view the world, ideas which have been created and followed by them naturally. A good example of this is that when presented with a picture of a lemon, a person will naturally, immediately connect it to the idea of sour, even if they've never tasted a lemon before. A picture of a lemon was used to prime the concept of sour into a real concept- although sour is still loosely defined as being both a flavour of food, and a personality trait, in which it then becomes abstract.
Less emotional abstract concepts such as time usually don't connect with any other magic type, although can still be combined with another magic type to cause some dangerous magical infusions. For example, a combination of time and dark magic: I'll leave the idea of what dangerous effects that could create up to you.
Any magic which is based off of an abstract concept in natural and human metaphysics can be categorised as Etherelium magic.