Maginulf was elected antipope Silvester in 1105 A.D.
The term antipope designated a person who strives to take the papacy from the duly elected pope. He usually gains some recognition but does not succeed in taking the papacy.
At his ascension to Maginulf took on the name Silvester, sometimes spelled Sylvester. The pope used his forces to kick Silvester and his followers out of Rome. A German king named Henry gave some support to Silvester while Henry had some conflict with the current Pope. Eventually, Henry and the Pope reconciled and dismissed Silvester, who died shortly after.
How many antipopes became popes? Winners generally write history.