First in a line of 14 popes

LEOI

This name belongs to a fifth-century pope who shares his title with a zodiac sign and was known for persuading Attila the Hun to retreat from Rome.

This clue refers to Pope Leo I, who was the first in a series of 14 popes named Leo throughout history. The answer "LEOI" is a shortened form, using "Leo" for the name and "I" as the Roman numeral for one, indicating he was the first. It’s a straightforward reference to papal numbering, where popes with the same name are distinguished by numerals.