O my Jesus, forgive us our sins, save us from the fires of hell, lead all souls to Heaven, especially those in most need of Thy mercy. Amen.
History
According to the testimony of the three shepherd children of Fatima, Our Lady taught them this prayer on July 13, 1917, asking that it be prayed after each decade of the Rosary. It spread with the Fatima devotion worldwide and is now part of how most Catholics pray the Rosary.
When to Pray
After the Glory Be of each Rosary decade — or on its own, as a short act of intercession for those furthest from mercy.