Mana

Mana is the essence of life in the air; it is produced by spirits of those long past, and is harvested by mages prodigal or otherwise.

History
Mana was originally discovered in 340 BE, nearly 3000 years before the scarcity of resources. Many empires of that time were revered for their research in the occult; despite Mana being literally the life essences of their ancestors, many diplomats couldn't resist the urge to harvest this to expend their powers. Of the original empires, Aznerathe was one of the few cities to survive.

Controversy
Considering the fact that Mana is the literal the essence of lives past gone, many found the use of magic to be controversial. Many figured that the use of Mana was disrespectful towards their ancestry, and many cared little. It was only a matter of time before entire empires were built around the use of magic; with that in mind, it was also only a matter of time before empires dedicated to the eradication of magic were formed. To this day, none of these empires survived. It wasn't until 3515, when the technological revolution was sparked that empires against magic were once again formed.