Emprēthō (ἐμπρήθω) means to set ablaze, to burn — a compound of en (in) and prēthō (to blow, to swell with heat). It appears in Matthew 22:7 where the king burns the city of those who rejected his wedding invitation.
Matthew 22:7 is the parable's darkest moment: "The king was enraged. He sent his army and destroyed those murderers and burned their city." This is most naturally read as a reference to Jerusalem's destruction in 70 AD — the burning of the city that rejected Jesus. Emprēthō carries the weight of divine judgment through historical agency: Rome's armies become (unknowingly) instruments of the king's wrath. Fire in Scripture is both judgment (Sodom, Nadab and Abihu) and purification (Isaiah's coal, Malachi's refiner's fire) — the same element with different outcomes depending on whether one is inside or outside the covenant.
The burning of the city in Matthew 22 bridges parable and history in a way unique to the Gospel of Matthew, written after 70 AD or incorporating prophetic material about it. The allegory doubles: the king is God, the servants are prophets, the son is Christ, the murderers are Jerusalem's leaders, the army is Rome, and the burning is the Temple's destruction. Emprēthō in this context is almost too precise — the Temple was indeed burned, the city torched.