What is Romeo's premonition in Act 1, Scene 4 of Romeo and Juliet?

Romeo has had a dream telling him that something will happen that night, which he assumes means at the Capulet masquerade party that he, Benvolio, and Mercutio are walking toward, torches in hand. Romeo fears that whatever happens will lead to untimely death. He has uneasy feelings of sadness and foreboding. Mercutio, ever high-spirited, does everything he can to quash the premonitions of the moping Romeo, pooh-poohing the idea that dreams might foretell the future....

Romeo has had a dream telling him that something will happen that night, which he assumes means at the Capulet masquerade party that he, Benvolio, and Mercutio are walking toward, torches in hand. Romeo fears that whatever happens will lead to untimely death. He has uneasy feelings of sadness and foreboding. Mercutio, ever high-spirited, does everything he can to quash the premonitions of the moping Romeo, pooh-poohing the idea that dreams might foretell the future. He calls them:

vain fantasy ... thin of substance as the air / And more inconstant than the wind

It is ironic that Mercutio dismisses Romeo's foreboding, for the unspecified "untimely death" Romeo has dreamed of foreshadows Mercutio's demise as well as his own. Going to the party sets in motion a chain of events that has the same dire result for Mercutio as for Romeo.

Romeo's forebodings add a shade of darkness, casting a shadow over a light-hearted, excited scene and reinforcing the play's theme that fate plays a strong role in the events that unfold.