Mystical power is usually somewhere in fantasy stories. Not always, but usually. It goes by lots of different names: thaumaturgy, the Force, the current, elements, but for our purposes, we'll go with the old stand-by and call it magic.
NUMBER TWO: Magic
If you're going to have magic, you need to have it be three things: logical, well-defined, and limited.
This seems counter-intuitive. It's magic right? Why should it be logical? It doesn't need to be logical like real science is logical, where you can explain every single detail and have it actually occur. That's no fun. by logical, I mean it needs to have rules, and that you shouldn't break those rules to make it convenient for the plot.
I used a term term in one of my previous posts, "Deus ex Magicka," which is a play on "Deus ex Machina," or "God from the Machine." When somebody mentions something as being a Deus ex Machina, they mean that it's a cheap, convenient way for a problem to be resolved in a story. this is a Very Bad Thing. It means the writer is lazy and uncreative and so just has some solution show up and fix everything.
Bad, illogical magic acts this way. Suddenly the hero knows how to cast fire for absolutely no reason at all and is saved or they conveniently break previously established rules in a way that becomes acceptable.
A good magic system with clear-cut rules is the one in Harry Potter. Rowling spent years thinking about it, and it shows. Things like wand movement, quality of a wand, pronunciation, and even the intent of the spellcaster are taken into consideration. There are multiple branches of magic as well (potions, dark arts, defense against the dark arts, enchantments, etc.) and each has their own rules. For potions, it depends not just on ingredients, but how you stir, how hot the mixture is, and so on. Every aspect is very well thought out and Rowling probably has tons of information she thought of that never made it into the books.
A bad magic system with very little in the way of rules is the one in Clive Barker's Abarat. Words of power are said, but they have random or vague effects, and it's unclear if any skill is required, or if somebody needs only to know the word to make magic happen. It's pointed out that expert magicians can create glyphs (magical vehicles from thin air) in a matter of minutes, while novices need more time, but it's never explained why. To summon a glyph, one need only recite a magic phrase over and over again. So what makes an expert need to say it less while a novice needs to say it more? It's never explained. Things just...happen.
Rules provide structure, and challenges. They make any magic system more interesting and complex, and more believable. Just remember to stick to your own rules.
You and the reader should both know what magic in your world means. How does it feel? What's it look like? The magic you see in the video game Skyrim is very different from the magic you see in The Dresden Files by Jim Butcher.
The magic in Skyrim is prevalent and in wide use and is incredibly easy for anybody to pick up to some degree. There is no shock or awe in seeing somebody conjure a fireball or turn invisible. Anybody can do it, literally. Especially if they have potions or scrolls or something. Magic in the world of Skyrim is cool, but it's really just seen as a type of science. It's an academic subject that lets you shoot lightning from your hand. Some people even comment that studying it is super boring and dull.
Magic in The Dresden Files is unheard of through most of the world. It's a secret that very few people have knowledge of. It's also not as convenient as it is in Skyrim. Not everybody can use it, and those who can use it, can't use all the different types of it. And you can't just fire off any spell willy-nilly. Some magic requires hours (or days, or years, or centuries) of time to set up and cast. It feels much more rare and special than in Skyrim, as well as more mysterious and dangerous.
Note that neither of these is bad. they're both fine, they just have very different feelings to them. Know what kind of magic you want in your story.
Finally, magic should have some kind of boundary. There should be some things that it definitely cannot do. Otherwise the solution to every problem is magic, and that becomes predictable, and that becomes boring. In the aforementioned Dresden Files spamming magic is impossible, because it's like physical stamina: there's only so much energy a practitioner can have. They can die if they use too much or overreach themselves. If they screw up a spell, it can backfire and kill them, or summon some creature from beyond that rips them apart.
Pretty much every magic system I've run across has some form of limitation, even the really bad ones. There are different ways to limit yourself though. Maybe some magic is just flat out impossible, like the genie in Aladdin saying he can't make anybody fall in love with Aladdin, kill anyone, or give Aladdin any more than three wishes. Or maybe some magic is so complex that it requires ultra-rare materials and incantations to pull off, and one wrong move means disaster. There's lots of different ways you could do it.
That's all I got for now. Next time I'll be talking about The Quest.
And yes, I'm aware I've been away for a month, but I've still been writing, so there.