To pray for the soul of someone is to pray for the salvation of that person, which means that he or she will have to repent and be reconciled to God. When all souls turn away from evil and to God, we shall have peace on earth. Therefore, I see praying for all the souls as the most fundamental prayer to change the world and to please God. Modeled after St. Gertrude's Prayer for the Holy Souls in Purgatory, I have come up with this succinct prayer:
Eternal Father, I offer you the most precious blood of your Divine Son, Jesus, for the salvation of all the souls, both living and deceased.