When Romeo says with this night's revels and expire the term of a despised life clos'd in my breast by some vile forfeit of untimely death he is foreshadowing?

Romeo is foreshadowing his own untimely death by suggesting that his life may end prematurely as a result of the events and actions that unfold during the evening's revelry. This sets the stage for the tragic turn of events that will ultimately lead to Romeo and Juliet's deaths.