Ability ConcentrationEntropy-
Shallowly defined according to physics (i.e. really just what's relevant for you):
An increase in the disorder/chaos of a system. Here are some examples relevant to this universe: the more particles can move, the more entropy they have (so gasses are highly entropic while solids are not). Also, the more entropy there is in a system, the less energy there is available to do what is defined as useful work. When a system reaches equilibrium its entropy is maximized. If a living system (person, tree, animal, etc) is at equilibrium that means it's dead. So death and destruction really is favorable to him and his abilities.
Defined Elementally:
Entropy as an element can best be defined as natural things/reactions happening in unnatural ways. A good example would be a tree dying and rotting, it is something you expect, but you don't expect it to happen quickly, or even instantaneously. Such is something natural happening unnaturally.
Magic manipulation (via entropy of course)-
Valerian can help increase the entropy of a localized system (as opposed to the universe which is THE system, though it too benefits from an increase in entropy) like he was exhaling a breath. Ice attacks can be converted to water or vapor to increase its entropic value. Decreasing entropy on a localized level takes more work (such as taking a fire attack and converting back to the carbon and oxygen it once was) and tends to have disasterous effects on the surroundings (and sometimes himself) as all of Valerian's magic
must contribute to increasing entropy of the universe. (It is impossible for him to do anything that would violate the law of entropy and, should he try, it would result in his immediate demise.) Eventually he hopes to bring about
the heat death of the universe but we won't get into that.
Reality distortion-
Thanks to the chaos fragment and his sagely abilities, Valerian is able to make his own little pocket dimensions rather than simply making a hammerspace to store items or a portal to another place in reality. The dimensions exist just beneath the surface of reality, much like a tumor would lie beneath the flesh of a person. The dimensions themselves are distorted in ways that only Valerian seems to understand. Their chaotic, atypical nature is what allows them to exist for any length of time, as a dimension that is alternate from reality but still ordered like it would be far too much of a strain to uphold. It IS called a chaos fragment after all...
MagicValerian's magic revolves around the concepts of entropy above and the manipulation of it in systems where systems can be defined as a magical attack, the environment around him, and even living things. Valerian cannot concoct any offensive magic of his own but rather, he manipulates others' magic to do that for him. How it is manipulated depends on the magic itself and a bit on the situation he's in. That is, the more disorder and chaos there is around him the easier it is to use his magic/the stronger his magic is.
Oversoul: Valerian has used his oversoul once, though he has no recollection of it due to the intense emotional distress he was going through at the time. His oversoul has the ability to literally tear at the fabric of reality, causing an area to become something along the lines of
Escher painting where the laws of the world seem to be broken and torn every which way. Eventually (the larger the area, the sooner), the place becomes so unstable it collapses on itself and explodes, akin to a supernova.