Literary devices are those devices that writers use to express and relay their messages and make their works better and richer. It allows the readers to better understand and gives them a better picture of the story. An allusion is one such device where the writer alludes to a particular story without explicitly referring to it.
In the given scene where Mercutio encourages Romeo to "<em>soar</em>" with "<em>Cupid's wings"</em>, Shakespeare is alluding to Cupid, the god of desire and love. This allusion seems to suggest that Mercutio wants Romeo to fly high and enjoy himself at the Capulet's party, and to look for love and not be depressed or sad.
Thus, the literary device that William Shakespeare used in this scene is allusion.
