Magic

Over the years, magic and technology have become intertwined, one building off of and feeding the other. Magic, however, is a naturally occurring phenomena, said to be born of residual energies from the Old Gods. 

SOURCE OF MAGIC

Souls are what fuel magic. Souls are a life force that can be found in nearly anything, from people, to plants, to entire planets. The soul creates a limited well of energy that can be used towards magic spells, which is also known as mana. Because of this, magic cannot be acquired, it can only be honed.
Souls are often described as a "well", the mana within is the water. Some souls are deeper than others, able to hold more mana. In rare few cases, there have been recordings of souls that are as vast an ocean.
To be able to perform magic is a genetic malfunction. Because of the dangers, it should not be possible. The "well" should be sealed shut, but all have at least a tiny crack in the door. It has been observed that magic users are increasing more and more, and blockers set in place are getting weaker.

Manifestation

To perform magic, willpower and visualization is utilized to create the change one wishes to see. This can be difficult, so spells are sometimes written or sigils are drawn to help one concentrate their intent. Those very in tune with a certain kind of magic may not need these. When magic is attempted, their intent may manifest into the physical reality.
Magic can manifest in a number of ways, depending on the spell and the mage. Some kinds of magic spells may appear as glyphs and symbols to help solidify them in the world, other spells may just result in a fireball or sudden frozen spikes.

LIMITATIONS OF MAGIC

Because magic uses energy much like any other activity, there is a limit to what one can do. It puts strain on the body just the same, such as running or lifting. Like physical energy, spiritual energy will replenish itself to its natural state, given time and healing. Low strain spells include divination, creating a small spark, conjuring a light, and levitating light objects close by. These types of spells will do little to no bodily harm, especially if they are doable without magic.
Moderate spells can be just about anything. Healing and conjuring elements are the most common ways that magic can manifest The highest level spells include reviving the dead, manipulating time, changing the construct of something. These things are only ever accomplished with powerful magical items, and even then, is so rare that it’s hardly been recorded in history, likely due to death being a guarantee. Using too many lower level spells in a short amount of time can have the same effect as using a higher level spell.

THE AETHER

Also known as the veil, the other side, the astral, the land of dreams. The Aether is a realm that exists beyond the physical, that binds magic and souls. It is in fact where souls reside, and many believe souls return there when a body dies.
Different cultures from across different planets have various understandings of the Aether, but they all tend to agree that Harbingers can inhabit both the spiritual world and physical world at once. It is a difficult place to glimpse into in order to  study, but the few times it has been done, the Harbinger's true, original forms can always be seen.