by St. Gertrude the Great
In purgatory, the souls of many of those who have died in God's grace undergo purification so that they may enter heaven.
The Catechism of the Catholic Church explains purgatory in paragraphs 1030-1032.
The Prayer of St. Gertrude, below, is one of the most famous of the prayers for souls in purgatory. St. Gertrude the Great was a Benedictine nun and mystic who lived in the 13th century. According to tradition, our Lord promised her that 1000 souls would be released from purgatory each time this prayer is said devoutly.
"Eternal Father, I offer Thee 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, those in my own home and within my family. Amen."