Explain this quote: "Good sir, why do you start , and seem to fear/ Things that do sound so fair?" What kind of literary device is it?

This was what Banquo said to Macbeth, on observing his startled reaction to the Witches' prophecies that were made on the deserted heath, after the 'hurly burly was done,' the battle was won.

The Witches' has prophecised Macbeth to become the Thane of Cowdor and the 'King hereafter', those were two promotions at the same time! These fortunate and good promotions were what Banquo said sounded 'so fair'. 'Why do you start, seem to fear' is a reference to his worried, anxiety-ridden expression, his thoughtful face, which was given rise to because Macbeth's ambitious nature had always dreamt of becoming the King of Scotland, but having the 'agents of devil' tell him that it was an actual possibility, was what shocked him beyond reality.