In this world, everything, swords, rivers, emotions, even lies, has a Resonance, an echo of existence. Certain people, called Harmonics, can attune to these Resonances and wield them like instruments.
Simple at first: fire has a Resonance (you can amplify flames, make them sing, bend them).
Deeper later: complex Resonances exist, like "betrayal," "dreams," or "silence," which can shatter armies or rewrite memories.
Power comes from understanding, not brute force, to master a Resonance, you must know its truth.