What rhetorical device is used in this quote: "A king before, but now a beaten warrior."?
A combination of contrast and chiasmus. There might be more.
Contrast: King Vs Beaten Warrior and Before Vs Now. The sentence combines opposite ideas to highlight the differences.
Chiasmus: The sentence structure is ABBA, or reverse parallel. If the sentence was parallel, then the sentence would (reversely) read, "Before a king, but now a beaten warrior."
Combined, both literary devices draw attention to the vast change that has occurred to this fallen king over time.