Pope Sergius III was pope from 897 to 911, and he is the only pope known to have allegedly fathered an illegitimate son who would later become pope, Pope John XI. John XI, a Roman, son (as some say) of Pope Sergius III, came to be pope in March 931 at the age of 20. He served until his death a mere four years later, a period that some historians consider the papacy's darkest hour.

Historians are split on who to call the father of John XI. Many agree with papal records and history, noted in the "Liber Pontificalis" that Pope Sergius III was John's father. If this is true, John XI is the only known illegitimate son of a pope to have become pope himself. However, some historians still maintain that John's father was in fact Alberic I of Spoleto.

More Info: en.wikipedia.org