The line of dialogue from Act III, scene I of Romeo and Juliet that most foreshadows that Mercutio’s death will lead to other tragic events in the story is "This day’s black fate on more days doth depend. This but begins the woe others must end."
Explanation:
These lines that Romeo says after the death of Mercutio, set the precedent of the future his beloved one and himself will suffer, not knowing how terrible the development of the tragedy will be he knows deep inside as he says in his words that this is just the trigger of all the terrible events that are about to happen to all the people involved.