According to history, Atilla made it to Rome and was about to take over when Pope Leo 1(I think that was his name) stopped him. How did a Pope convince a Hun to leave? There are three possibilities.
One: He impressed the Hun with his piety and claimed that Saint Paul helped him. This seems unlikely, since piety didn't stop the Hun from destroying churchs elsewhere? Also, I doubt Saint Paul helped him. Besides, I doubt he would help the empire that had him beheaded.
Two: The Pope used Atilla's superstition against him. Let's remember that while Atilla was aethistic he did have many superstitious beliefs. He was known to have a band of shamans and magicians. The Pope might have told him that the spirits of the saints would be angry if he took Rome.
Three: He used political savvy to get Atilla to leave. This is the most likely. There was a plague going on in Italy at that time, so Attila feared that his victory would be undone if he took a plagued city. Also, Rome had been sacked several times and was in ruin. The Pope might have convinced Atilla that Rome was not worth taking for it was in such ruin. Atilla didn't care about money, he cared about conquest, so the Pope could not give him money to leave.
Somehow, the Pope convinced Atilla to leave. Atilla probably thought that he would come back later to take Rome. Unfortunately, he died of suffocation by a nosebleed so that didn't work out. Comments and corrections are welcome.