"The entry into Rome proper took place on 23 December [1655], on horseback through Porta Flaminia, which today is known as Porta del Popolo." - Wikipedia about Queen Christina of Sweden
In the year 1654 Christina abdicated as queen of Sweden, became a Catholic and spend the rest of her life in Rome as guest of the pope.
Your challenge is to have Christina, daughter of Gustav II Adolf the Defender of Protestantism, to sack and burn Rome in the December of 1655.