The story is that Pope St Leo I met Attilla and persuaded him to not sack Rome it was 300 years later that the tribes of present day Germany, including the Huns were converted. Below is a pertinant section from the Catholic Encycopedia on the Conversion of what is now Modern Germany, Holland, Belgium and North Eastern France.

Among the early missionaries were: St. Columbanus, the first to come to the Continent (about 583), who laboured in Swabia; Fridolin, the founder of Saeckingen; Pirminius, who established the monastery of Reichenau in 724; and Gallus (d. 645), the founder of St. Gall. The cause of Christianity was furthered in Bavaria by Rupert of Worms (beginning of the seventh century), Corbinian (d. 730), and Emmeram (d. 715). The great organizer of the Church of Bavaria was St. Boniface. The chief herald of the Faith among the Franks was the Scotchman, St. Kilian (end of the seventh century); the Frisians received Christianity through Willibrord (d. 739). The real Apostle of Germany was St. Boniface, whose chief work was in Central Germany and Bavaria. Acting in conjunction with Rome he organized the German Church, and finally in 755 met the death of a martyr at the hands of the Frisians. After the Church had thus obtained a good foothold it soon reached a position of much importance in the eyes of the youthful German peoples.

Never heard that story. But that does not make it untrue. I’ll look around.