Final answer:
In 'Romeo and Juliet', Mercutio curses both the Capulet and Montague houses with "A plague on both your houses" after being mortally wounded in a sword duel by Tybalt.
Step-by-step explanation:
The character in Shakespeare's Romeo and Juliet who says, "A plague on both your houses" after losing a sword duel is Mercurio, known in English as Mercutio. He is a close friend to Romeo and is known for his witty and somewhat cynical view of love and life.
Mercutio is mortally wounded by Tybalt, a member of the Capulet family, and with these famous dying words he curses the Montagues and the Capulets, highlighting his disgust at the families' ongoing feud which ultimately leads to his death.