In real life, magic was how people used to try and bend the laws of nature back when those laws seemed too hard to decypher and master. It's no coincidence that in many old cultures magic and religion were bound together if not the same thing. And gods were fickle.
That last part has been preserved in fiction, where magic plays the role of a powerful force, able to grant a huge advantage, at the risk of running out of control and doing more harm than good, never mind the sacrifice it requires in the first place. Any resemblance with modern technology is deliberate.
In computer games, it's more tricky to capture the chaotic nature of magic, or for that matter to keep it fair. Moreover, it's hard to make up a magic system outside of a given world. However, I have one you can plug with relative ease into an ISO Standard Fantasy Setting.
The goal of this system is to limit how much players can rely on magic without introducing yet another concept, like mana, and to ensure magic doesn't overlap too much with other game mechanics, lest it become pointless
Most people encounter magic in the form of spell scrolls. These are written in a magical language, but the script is usually mundane, so that anyone literate may use them. Reading the spell out loud, or signing it, triggers the effect. (Some wizards insist that doing both at once is more powerful.) Due to magical backwash, the scroll crumbles to dust at this point, so each of them can only be used once.
Now comes the fun part: after casting from a scroll, anyone can try to remember the spell and cast it again from memory. To do that, the character must first succeed at a static Focus check against a target number equal to the spell level times ten. That takes time equal to one game round (see the section on time tracking) per spell level; if using real time, make one round equal to ten minutes. A failed check can't be retried until after a number of hours equal to the spell level. A successfully remembered spell can then be cast as normal, within a reasonable amount of time, say the same day.
Some spells can be memorized at a higher level than their stated minimum, with a corresponding increase in difficulty. When cast, they'll have proportionally bigger effects and longer duration. Scrolls however have fixed spell levels.
Either way, casting a spell temporarily blanks the caster's mind, so they can't attempt to cast the same spell again right away unless they have a scroll for it handy. They can still cast any other spell they have memorized.
Last but not least, spells tend to have catchy one-word names.
Level: 1+; duration: instant; range: caster.
Grants the caster one blessing per spell level.
Level: 2+; duration: 1 minute/level; range: 3 meters/level.
Strong vines sprout from the ground in a three-meter radius (per spell level) around the caster. They grow rapidly and forcefully, pushing upwards anything they can't grab and hold down instead. (The exact effect is sensitive to context.) The vines can sprout from all kinds of surfaces, breaking them if necessary, but die out soon unless they find soil to catch root in. Either way, they cease being magical once the spell has expired.
Level: 2+; duration: 10 seconds/level; range: 15 meters/level.
A strong wind picks up around the caster, and blows forcefully in the direction they're facing, spreading out in a 45-degree angle. The wind is strong enough to blow creatures off their feet or equivalent unless they succeed at a static Muscle check against a target number equal to the spell level times ten.
Level: 2; duration: continuous; range: caster
Whatever armor the caster is wearing starts glowing with a bluish light, and becomes resistant to fire damage as per the section on special effects. Conversely, it becomes vulnerable to cold damage. The effect lasts until replaced by a different enchantment, or until the armor falls apart.
Level: 2; duration: continuous; range: caster
Whatever armor the caster is wearing starts glowing with a reddish light, and becomes resistant to cold damage. Conversely, it becomes vulnerable to fire damage.
Level: 3+; duration: 10 seconds/level; range: 15 meters/level.
A cold wind picks up around the caster, and blows snow in the direction they're facing, spreading out in a 45-degree angle. The wind isn't especially strong, but anything in its path takes 1d4+1 of cold damage per spell level.
Level: 3; duration: continuous; range: touch
Whatever weapon the caster is touching starts glowing with a reddish light. Any damage it deals from that point on is treated as fire damage.
Level: 3; duration: continuous; range: touch
Whatever weapon the caster is touching starts glowing with a bluish light. Any damage it deals from that point on is treated as cold damage.
Level: 4+; duration: instant; range: 3 meters/level.
A tall flame bursts from the ground in front of the caster, then surges forward as if carried by a strong wind, spreading out in a 45-degree angle. The flame is briefly-lived, but hot enough to ignite paper or light fabric on the spot; anything else in its path takes 1d4+1 of fire damage per spell level.
Level: 4+; duration: 1 minute/level; range: caster.
The caster lifts into the air, about knee-high to waist-high, and can move around at up to their usual running speed, along with everything they can carry. They can maneuver normally while in the air, but not make sudden moves such as jumping aside to avoid an incoming missile, unless they push against a wall or some such.
Potions are quasi-magical brews sold in small flasks or vials. They boost one attribute of whoever drinks them for a short duration. Specifically, for the next three game rounds the character in question rolls one extra die for any roll involving that attribute, inside or outside of combat. Drinking another potion of the same type before the previous one has all but worn off extends the duration by another two rounds, but deals 1d4+1 of damage in the process.