According to tradition, St. Gertrude the Great was told by Our Lord that the following prayer, each time she piously recited it, would release 1000 souls (or a vast number) from their suffering in purgatory.
Eternal Father,
I offer You the most precious blood
Of thy Divine Son, Jesus,
in union with the Masses said
throughout the world today,
for all the Holy Souls in Purgatory,
for sinners everywhere,
for sinners in the universal Church,
for those in my own home,
and in my family. Amen.