Here’s what you should think about when developing your magic system.

• Why do people use it? Viewing magic as a process, why do people want to use it? How do they use it to do something in your story that they couldn’t otherwise?
• How does it work? This is the transmission layer. By what mechanism does the magic do the thing it does? It’s perfectly okay to say ‘by the blood of dead gods spilled into the ethereal seams of the world’ but I like it when there’s some thought behind it. Even if the characters don’t know, stick this thought in your 90% of worldbuilding that the reader will never see. It’ll help for background consistency.
• What does it feel like to use magic? I love stories where people are exploring their powers (I enjoy superhero origin stories, except those we’ve seen repeatedly; looking at you Batman, Superman and Spiderman). How does it feel to channel and cast power? Anxiety of trying to memorise a difficult formula? Getting high from channelling raw energy from the gods? Is there a taste or sensation? Or even boredom, if magic is perfunctory?
• Who can use it? Trained wizards? Anyone who gets the spell right?
• Where does the magic fit into your world and society? Is it a secret? Only used by the elite?

Does your magic have an overall paradigm? Like a special esoteric programming code (spell) that can hack reality can if done right? Calling upon ancient gods for boons? The flavour is important to me. I read the first few pages of a book where the hero ‘magicked a barrier in front of the demon’ and while the scene was action-paced, the flavor of the magic didn’t grab me.

Let’s run my magic system through these questions:

• Why do people use it? To do things they can’t do via ordinary mortal means. Because it requires making a pact, it’s all for personal gain or desperation. Maybe to help with revenge, or to return after death to deal with your unfinished business.
• How does it work? Magic is a flow of energy from another dimension. A flow of extra-dimensional energy overwrites the localised reality, enabling supernatural effects when present. For example, to summon a zombie, you’ll need a source of spectral energy from the Underworld, the land of the dead.
• What does it feel like to use? Each realm has a distinct flavour of energy. Infernal magic is painful, like barbwire running through your guts. Death magic is sad and regretful, like holding a party that no one shows up to.
• Who can use it? After the Rending—the terrible event when the Age of Magic ended—all portals to other dimensions were abruptly sealed off. Demons, fae, nature spirits, angels are trapped in their home realms and have limited agency to influence the mortal world. However, if you make a pact with one, you gain their vestige—a shard of their soul—and this enables you to channel supernatural energy into the mortal world. This changes you—you’re not a normal mortal anymore. You’re now half an extradimensional entity. Someone who accepts a demon’s vestige becomes a cambion; another who makes a pact with a fae becomes a changeling.
• Where does the magic fit into the world? It’s secret and hidden. You have to figure out that magic exists, who you want to make a pact with, and hopefully find a patron whose goals align with yours.

The overall vibe is if you want magic, you hustle for it, and cut deals with powerful extra-dimensional entities. It’s a grungy, noir occult world. You take on supernatural debt and have to weigh the bargain you’ve made against the power you gain. Sometimes you may not have a choice but to agree.

“So everyone’s a D&D warlock?” someone asked when I described this.

Yeah.

Or John Constantine, as you sit on a teetering mound of debts and favors that are gradually spiralling out of control…

How about you? How does your world’s magic work?