Answer:

leoi

AnswerCrossword Clue
LEOIFirst of thirteen Popes
LEOIPope called "The Great"
LEOIOne of 13 like-named Popes
LEOI"... know it's more common to clue this as the 5th-century pope, but it seems easier this way; don't you agree, Mr. DiCaprio?"
LEOIFirst in a (non-contiguous) series of thirteen popes
LEOIPope who repelled Attila the Hun
LEOIFifth-century pope
LEOIPope after Sixtus III
LEOI5th-century pope
LEOIPope from 440-461 A.D.
LEOIPrimary of 13 popes
LEOIFirst of 13 popes
LEOIFifth century pope known as "the Great"
LEOISainted fifth-century pope
LEOIPope during Attila's time
LEOIFifth-century pope known as "the Great"
LEOISainted pope called "the Great"
LEOIPope before Hilarius
LEOI5th century Pope known as "The Great"
LEOIPope who convinced Attila to spare Rome
LEOIPope from 440 to 461
LEOIPope nicknamed "the Great"
LEOI"The Great" pope
LEOI5th century pope
LEOI"The Great" saint
LEOIFirst "Great" pope
LEOIHe became pope in 440
LEOIPope who persuaded the Huns not to attack Rome
LEOIPope with a dozen namesakes
LEOIPope before Hilary
LEOIPope of the 400s
LEOIByzantine emperor, 457-74.
LEOIGreat early pontiff, 440-61.
LEOIGreat pontiff (440-61).
LEOIGreat pope (440-461).
LEOIEarly pontiff.
LEOIPope, 440-461
LEOIPope who became a saint
LEOI"Great" fifth-century pope
LEOIA pope who became a saint