The Primordial Aspects

The Primordial Aspects, or the Aszarei, are four symbiotic magical entities. Each Aspect is sentient and possesses its own personality, but they are very primitive and primal. The Aspects are not of this world, and are comprised of magic that is fundamentally and structurally different to anything else in existence. Because of this, each one is apocalyptically powerful, and has the power to tear reality asunder. In order to be used at their full potential, an Aspect must bind to a mortal host.

There are no details of the Aspects' appearances in history, making their existence extremely difficult to prove save for the destruction they are leaving in their wake. The four Aspects are Famine, War, Death and Time.

(tl;dr, they're basically Infinity Stones)

Origins
The Primordial Aspects were created by the Rainmaker, the Aspects were designed to be 'gifted' to four mortals who had been trained vigorously and would be able to control such power. These four mortals, known as the Four Horsemen, would then bring the world under heel for the Rainmaker. The motives for doing so are unknown, and the exact moment in history when this occurred is also unknown as no proof of the Rainmaker's existence can be sourced.

Dalton Wood, a lich formerly in service to the Rainmaker, is the only person who seems knowledgeable on the Aspects usage. Every recent appearance of the Aspects in the last five years can be traced back to him, with the logical conclusion being that -he- is the Rainmaker. However, it's been confirmed that Dalton in fact stole the Aspects from the Rainmaker and fled, and is now experimenting with them for unknown reasons.

Usage
A Primordial Aspect is an immaterial possession that exists either within a mortal being (or the 'host') or stored within a magical artefact. Only witch doctors with extremely specific training can move an Aspect from one host to another. If the host of an Aspect dies, the Aspect itself will either remain within the host's lifeless corpse or retreat to the nearest source of magic and store itself there.

A Primordial Aspect can be 'tapped' by skilled mages. A magic user who was in possession of an Aspect that was stored within an artefact could extract its power for all manner of uses, depending on which Aspect it was. They could, for example, build an entirely new school of magic or create super-soldiers. Dalton has previously used the Aspects as a toolkit for all manner of research. Tapping an Aspect is both extremely dangerous and unpredictable, often driving its user insane.

The Four Aspects
A more detailed look at each Aspect, what they can do and where they have appeared so far:

Death
The Death Aspect is the only Aspect to have been properly imbued into a host; Finnick. At some point, Finnick's father Maxwell began performing experiments on the Aspect of Death after being coerced into doing so by Dalton. Finnick was a 'test subject' which Dalton had no way of controlling, and Dalton fled to safety when he realised he couldn't contain him. When Finnick's father was killed and the boy was set free, he tried to use the Aspect to tear the wealthy and powerful from their thrones violently, in a misguided campaign that left thousands dead. He was stopped by a select few individuals from Guildereim. He is currently a prisoner, and remains the true host despite having his powers nullified.

The Aspect of Death itself is a gateway to control of life itself. This includes the physical side of it, such as flesh, bone and blood, but also the spiritual side such as the mind and soul. The Aspect of death can be used (ironically) to create life as well as destroy it, and to control or synthesise chemical compounds and structures. Entire armies of constructs can be built in the snap of a finger, villages and cities can have their populations reduced to insanity or dust, powerful magical viruses and magical afflictions, such as wereblood or vampirism, can be cured instantly. It is best suited in the hands of necromancers or druids; these would find themselves becoming nigh unstoppable when using the Aspect of Death.

War
Dalton has used to Aspect of War repeatedly as a toolbox for experimentation. He has created super-soldiers (though not with much success) and, even know, continues to try and repeat his work. Dalton has recently started working closely with Saron Deimos, a reputable warlord with high ambitions. Though the nature of their alliance is unknown, it seems likely the Aspect of War is involved - how deep that particular rabbit hole goes is yet to be revealed.

The War Aspect is unique amongst the others in that it doesn't appear to be outwardly magical. It is, however, the most ferocious and destructive of all three. The host of the War Aspect would be enhanced to their absolute psychical and mental peak; they would be strong, fast, supremely intelligent and unfaltering cunning. It can also imbue beings with properties they did not previously possess, such as supreme intelligence or indestructible skin. Unlike the other Aspects, the War Aspect will destroy 'everything' it touches indescriminately. The host of the Aspect of War is always consumed by it; those possessing it are compelled to commit acts of violence and conquest. Some Warlords might covet this, as they become unstoppable and crush their enemies, but the War Aspect would then force them to turn on their own kingdom and beloved allies, tearing everything apart until they have utterly self-destructed; there are no exceptions to this. Those attempting to exploit the Aspect of War lose themselves to it. The War Aspect can also be used indirectly to create super-soldiers, genius level intellects or imbue entire armies with unstoppable fury.

Famine
The Famine Aspect has not made a definite appearance just yet. Since Dalton hasn't performed any experiments with it, it's likely he either does not wish to risk unleash its power (having learn his mistake from Finnick) or is showing more caution that he would with the War and Death Aspects. This leads one to conclude that the Famine Aspect is particularly destructive. It is likely that the Famine Aspect is not currently possessing a host, but rather has been stored somewhere for safekeeping.

The Famine Aspect is elemental in nature, almost acting as a flipside to the Death Aspect's control of life and soul. It gives its host control over earth, fire, wind and water; anything inorganic. Summoning natural disasters or interfering with climate, the user can shape and manipulate the balance of nature to their favour whilst those caught in their wrath can simply try and survive.

Time
The Time Aspect remains the most mysterious and unknown of the Aspects. There's no sign that it has ever been used, tapped or otherwise shown to exist. Chronology, the theoretical field of magic involving the manipulation of time, is largely a myth. Whilst the concept of time travel is, to some, an interesting prospect it's also a physical impossibility. The past is written; the ink is dry. Were one to travel backwards in time and interfere with history, now or in the future, the effects of their meddling have already been made - the past cannot be changed.