A Path of Endless Crossroads. The Tipping World, #1

No one but a mage can kill a mage, according to the saying. And now they're vanishing. Seven hundred years ago, war wracked Boscari. From the ashes rose a new system, peace and trade its mandates. Its enforcers: mages, possessed of uncanny martial skills and the ability to change the world around them just by thinking. At the war's end, they swore to dedicate these gifts to serving others. Alia grew up isolated in the countryside and knows little about this history; she takes more interest in the wild hearthside tales of her family's cook, adventures peopled by fantastical creatures like mula and mages.
When a real specimen shows up at her door, she's less amazed by him than by the facts he carries with him: her parents were exiled from their homeland for denouncing a mage for murdering people. To learn they exist and don't always live up to that promise they made is concerning since her parents ask the man to train her. At the Complex, she finds a society falling apart. Mages' ruling council is locked in conflict over a dispute that has more to do with egos than policy, and paranoia flourishes: people's friends are going missing, they're right to be afraid.
No one but a mage can kill a mage, it's not a tricky riddle, but who the traitor is and what that person wants isn't obvious. They already rule the world. Alia and her friends settle on a reckless quest to challenge the villain, but they're about to learn that nothing is that simple. Especially not the enigma called the Mage-Killer.