Aether
Aether is the foundation of all life and magic on Hydaelyn. Aether is found in all living beings and is an essential building block of creation. The various schools of magic have learned to draw upon aether and use it to empower their spellcraft. The study of Aether is known as Aetherology.
Life
Through all things—people, animals, plants—aether is observed to flow. It is a current that gives life and energy. When something dies, aether no longer passes through it. Living beings will slowly lose aetheric density as they grow older. When food is consumed, the body converts some of that energy into aether, sustaining life on multiple fronts. While aether flows through everything, those currents can also be stopped. Scholars liken this to the "atrophication of a limb cut off from the heart". Without sufficient aether, the people and the planet would perish.
Magic
manipulating aether that deviates from its natural tendencies is what scholars call Magic. Though the effect achieved by various schools of magic such as Thaumaturgy and Conjury are vastly different, the method employed is essentially the same. Scholars have observed a more primitive form of aetherial manipulation in lesser creatures, such as a Raptor breathing fire or a Biast unleashing a levin-bolt are the most basic forms of aetherial manipulation found in nature. This form of "magic" utilizes aether from their bodies or the surrounding environment and is used to hunt or protect themselves from harm. Some who study the arcane arts use specific items or tools to strengthen their magic or focus their aether—Arcanists and their Carbuncles for example.
Elements
Aetherial energies are divided into six different elements-Fire, Wind, Lightning, Water, Ice and Earth. These elements dictate how things will behave and interact with one another in nature. For instance, areas that are more "Fire-aspected" will have hotter and drier climates than those that are Water or Ice-aspected. This will also affect the flora and fauna of an area as well, affecting how a being can navigate the world, with Wind-aspected creatures being able to soar through the skies, or Earth-aspected beings having hardy scales, like the scalekin.
In addition to these elements, there are also two poles: Astral and Umbral. These poles also affect the nature of the elements, with Astral alignment being a more active form of energy, and Umbral being more passive. Astral Fire exhibits high volatility, where as Umbral Fire will generally affect dryness.
Naturally, some elements exist in a naturally Astral state, while others exist in a naturally Umbral state. These inter-elemental relationships help give balance and maintain each other. Elements in the naturally Astral state are Earth, Lightning and Water. The naturally Umbral elements are Fire, Wind and Ice.
Crystals
When living beings die, the aether within them will eventually flow back into the Lifestream and continue circling the world in the aetherial current. However, if a living being suffers a mortal wound, that life energy will sometimes instead pool and pass over into the corporeal realm, manifesting as a crystal. This can be a damaged piece of nature, a living being, or even the land itself. These crystals will become elementally aspected and store a massive repository of that element, which can then be used in any manner of ways. Fire-aspected crystals are often used in smithing for their ability to increase the temperature of forges. Given the intense concentration of aether in these crystals, consuming one will drastically alter the elemental and aetherial balance within the body, most likely leading to death.
Aetherologists believe that existence is made up of two different planes: the Corporeal realm, where all things take a physical form, and the Aetherial realm, a nonphysical realm without substance, but contains the essence of all creation. When something dies in the Corporeal realm, its aether passes between realms and back into the Aetherial realm, where it enters back into the Lifestream and continues its flow around the world. When new life is born into the Corporeal realm, it is granted energy from the Lifestream, creating a balance between the two realms. Each plane affects the other, and their proximity to one another dictates the scope of influence. When the two planes are close to one another, the Corporeal realm benefits by becoming rich in "Ambient aether"-aether richly charged with life aspect. In places where the realms are distant, the surrounding area generally has a harsher climate and is devoid of life. The planes are tied together in such a way that if one is damaged, the other would suffer as well, and if one is destroyed, the other would fail as well.
The Aetherial realm is also influenced by being in close proximity to the Corporeal realm by allowing for the maintenance of corporeal will when in a state of pure aether. Taking advantage of this, beings of the Corporeal realm can take advantage of this and use the Lifestream to travel between various points through teleportation. Magic is used to reduce a being to aether and pass into the Aetherial realm, and move from place to place. Aetherytes, which are aetherically charged crystals, are used as beacons of sorts directing the being where to go. The movement is almost instantaneous. These Aetherytes form naturally at the locations where the realms are closest to one another, and are equally surrounded by life as civilizations tend to crop up with them at their center. Without these beacons, entering into the Aetherial realm would simply cause a living being to enter back into the Lifestream, slowly breaking down into the basest components of life and continuing the cycle.
Hydaelyn
It is a widely held belief by scholars that somewhere deep within the planet rests a mass concentration of life-aspected aether, and have named this phenomenon the Mothercrystal. The belief goes further by stating that the Mothercrystal indicates the planet is a living being and represents a biological entity. As evidence to support this theory, scholars point to seers who have incredible visions of the future, or those who gain incredible strength in times of peril, and state the phenomena are actually the planet conveying its will to help those who protect it. They refer to this as Hydaelyn's Will.
Aetherial Sea
Sharlayan scholars constantly study the planet in order to predict its fate. They have observed that the further down towards the core one ventures, the more blurred the lines between the two planes becomes. It is believed that if one traveled far enough down, they would be able to observe the Aetherial realm while maintaining corporeal form. To this end, the Sharlayans built the Antitower deep underground to explore this hypothesis. However, before they were able to make contact with the Mothercrystal, they fled back to Sharlayan. Later, this hypothesis would be tested and confirmed when the Warrior of Light entered into The Aitiascope in order to entreat with Hydaelyn. They were fully able to reach the Mothercrystal and survived the Aetherial realm intact.
Primals
While Arcanists have devised methods to call upon semi-sentient familiars using their own aether, this is not the only method of summoning available. Another method involves the combining of corporeal aetherial energy with spiritual aetherial energy in the form of prayer in an elaborate summoning ritual to bring forth a physical manifestation of an ideal in the form of a god known as a Primal. This method of summoning has become increasingly common amongst the beast Tribes of Eorzea, who use this power to enforce their will upon the land and those who would stand in their way. These beings are also known as Eikons, a name derived from records of the Allagan Empire. When summoned, these beings will protect or empower those who have called them forth. They can also impress their will onto those with weak wills through using their own aether. This method of control is known as Tempering and was until recently thought to be incurable. This is due to the way the aether of the primal corrupts the aether of their victim. The problem with these summonings is that even if they are brought low, they can be quickly recalled via another summoning ritual. Repeat summonings can have a drastic effect on the aether of Hydaelyn and can reduce the surrounding area into a wasteland.