The fastest way to drain tension from a fantasy story is to let magic solve problems without cost. If a spell can fix anything and the only price is "the wizard gets a little tired," readers stop worrying, because they sense the story has an escape hatch. A magic system starts feeling real the moment it behaves like a law of physics instead of a wish-granting device — consistent, limited, and indifferent to how badly the hero needs an exception.
The most useful worldbuilding question isn't "what can my magic do?" It's "what does my magic take?" Time, memory, years of life, social standing, sanity, a physical material that's becoming scarce — the cost matters more than the power, because cost is what generates difficult decisions. A fireball spell is boring. A fireball spell that ages the caster a year every time it's cast is a plot.
Some stories use magic as atmosphere — strange, mostly unexplained, there to create wonder and dread rather than to be solved by the reader. Other stories use magic as a system with clear rules the reader can learn and predict, almost like a puzzle the protagonist has to out-think. Neither approach is better, but mixing them by accident is where systems fall apart — if your magic is mysterious in chapter two and suddenly behaves like a rulebook in chapter twenty so the hero can win, readers will feel cheated rather than satisfied.
What a system of magic cannot do, under any circumstance, is usually more dramatically useful than what it can. A healing magic that can mend any wound except ones inflicted by someone who loves the caster creates instant story potential. An unlimited healing magic creates a problem you'll spend the rest of the book working around.
A magic system that costs years of life will produce a society with different attitudes toward aging, inheritance, and ambition than one with no cost at all. Let the price of magic ripple outward into law, religion, and economy — that's usually where a magic system stops feeling like a mechanic bolted onto the plot and starts feeling like a real feature of how the world works.
Before drafting your next chapter, try writing one sentence: "Magic in this world costs ___, and the one thing it can never do is ___." IARobo's World Builder is built to take exactly that kind of constraint and grow a coherent system, religion, and culture around it.
Want to put this into practice? Open IARobo and try it in the Novel Generator, Poetry Studio or Character Creator.