When we look at our world, or universe, there are many unexplained things. We tend to attribute these unexplained things as 'magic'. Long ago wrapping a wound in a particular plant pulp would help heal the wound, and that was called 'magic', done by a 'witch'. Now days we call those witches 'doctors'. There were even witchdoctors.
Once it is understood, the science looses the term magic. However, as long as the mass majority of people do not understand how it works, then it is still 'magic'. So, for example, how did our universe begin? Well, most scientists believe there was a big bang... fine. What started the Big Bang? What caused it to happen? What came before it, because for something to start, something had to start it. Nobody has a widely accepted theory on that... unless you figure in religion, which, in essence is the belief in magic. A supreme deity fits quite nicely into the definition of magic, let's try it out: God is... a mysterious or extraordinary quality or power. Yep, magic.
So, when creating a fantasy world, I like to go back to the beginning... of everything. At least, the beginning of everything in the universe of this fantasy world (which has nothing to do with our reality, generally). I like to understand how things work, and from that, come up with a somewhat logical system within that framework. So how is this then 'magic'. Well, I said that 'I' understand it, not the people who live in that universe. To the masses in that fantasy world, there are unexplained happenings, and they call these things, magic.
Once I have an understanding of my universe I can set up a clear set of rules for my system of 'magic'. Now, this could be spells, I have nothing against them, I would just want to understand what is behind those spells, what is the force, and how do a string of words control that force and shape it?
In my NexLord series, the essence of magic came from the power of emotions and belief. In Cathexis magic comes from three completely different sources (there is no limit to what people find mysterious). The first was the property of the metal Cathexis, and how it can store a copy, or imprint of a person's thoughts and identity. The second was their aura. I got this idea from an old type of photography called Kirlian. To quote from Wikipedia:
"Kirlian photography refers to a form of photogram made with voltage. It is named after Semyon Kirlian, who in 1939 accidentally discovered that if an object on a photographic plate is connected to a source of voltage an image is produced on the photographic plate.
Kirlian said that the image he was studying might be compared with the human aura. An experiment in evidence of energy fields generated by living entities involves taking Kirlian contact photographs of a picked leaf at set periods, its gradual withering corresponding with a decline in the strength of the aura. In some experiments, if a section of a leaf was torn away after the first photograph, a faint image of the missing section would remain when a second photograph was taken."
I was intrigued by this, and decided that I would base part of my 'magic' system on this phenomenon, though I took my own liberties with the concept since this is fiction. As a tribute to Semyon Kirlian, in my books the school and practitioners of aura powers are called the 'Kir'nath, and the mercenaries also adopting some of those practices are the Tchu'lians'. Put the first and last parts together and you get, 'Kirlian'.
The third system of 'magic' is that which the Necromancer's use. For this I created a second universe, one where a kind of negative energy that had sentience fed on human souls (auras). Since they fed on life, I made their power that of death, and thus called those who tapped that power, Necromancers, since that term is derived from 'necro-' which is a combining form meaning “the dead,” “corpse,” “dead tissue.” While 'Necromancer' is defined as a noun:
a method of divination through alleged communication with the dead; black art
magic in general, especially that practiced by a witch or sorcerer; sorcery; witchcraft; conjuration.
And so I had my magic of life, 'Kirnath aura magic' and my opposite power of death 'Necromancers', and my strange metal, cathexis, which is also loosely tied into the Kirlian photography since inanimate objects like a coin, also showed this 'aura' at times: hence, the second book title, Conspirator's Coin.
So there you go, magic, and how I generally figure out the magic system for my books.