This is a humorous one!
During audiences with the pope, it’s common for them to impart a papal blessing at the end. They can also give a blessing by letter or parchment.
No particular form of words are required, but in the past some have opted to use the pontifical blessing that any bishop can use – the Dominus vobiscum, meaning The Lord be with you.
However, popular pious legends tell of some times when popes gave a more… fire-charged blessing.
As the tale goes, an anti-Catholic journalist attended a papal audience of Pope Pius XII and provocatively asked the Holy Father for his blessing. Without skipping a beat, he used this blessing:
“Ab illo benedicaris + in cuius honore cremaberis. Amen”
“Be ye blessed by Him + in whose honor thou art to burn. Amen”
Another story goes of an Anglican clergymen asking for a special papal blessing from Leo XIII after making a heretical utterance. The Holy Father replied he cannot give a special blessing to a protestant, but after their insistence he blessed them in the form above.
That same story is also attributed to Pius IX, but no matter the pope that gave it, it’s surely a good example of Catholic humor and charity!