Now Attila, having once more collected his forces which had been scattered in Gaul [at the battle of Chalons], took his way through Pannonia into Italy. . . To the emperor and the senate and Roman people none of all the proposed plans to oppose the enemy seemed so practicable as to send legates to the most savage king and beg for peace. Our most blessed Pope Leo -trusting in the help of God, who never fails the righteous in their trials - undertook the task, accompanied by Avienus, a man of consular rank, and the prefect Trygetius. And the outcome was what his faith had foreseen; for when the king had received the embassy, he was so impressed by the presence of the high priest that he ordered his army to give up warfare and, after he had promised peace, he departed beyond the Danube.
Later legends based on this event told of Ss. Paul and Peter coming to Leo's aid; this is the basis for Raphael's famous painting of the scene. There's a YouTube video (very uneven sound quality, and silly, but rather fun) about the event, using Legos and Carmina Burana.
Leo's most important theological work is the Tome to Flavian, which became one of the key texts of Chalcedonian Christianity. You can also read some of his letters and sermons (and more at CCEL).