The future Pope Victor II was born Daufer in 1026 or 1027 to a branch of the Lombard dukes of Benevento as a son of Prince Landulf V of Benevento.

After his father died in battle with the Normans in 1047, he had fled from an arranged marriage and, though brought back by force, eventually fled again, this time to to Cava de' Tirreni, where he had obtained permission to enter the monastery of S. Sophia at Benevento, changing his name from Dauferius to Desiderius.

Finding life at S. Sophia insufficiently strict , he had gone first to the island monastery of Tremite San Nicolo in the Adriatic and in 1053 to some hermits at Majella in the Abruzzi.

About this time he was brought to the notice of St. Leo IX, and it is probable that the pope employed him at Benevento to negotiate peace with the Normans after the fatal battle of Civitate.

Desiderius had later attached himself to the court of Pope Victor II at Florence.

There he met two monks of the renowned Benedictine monastery of Monte Cassino, with whom he had returned in 1055.

Joining the community, he was shortly afterwards appointed superior of the dependent house at Capua.

In 1057, Pope Stephen IX, who had retained the abbacy of Monte Cassino, came to visit and at Christmas, believing himself to be dying, ordered the monks to elect a new abbot.

Their choice had fallen on Desiderius.

The pope recovered, and, desiring to retain the abbacy during his lifetime, appointed the abbot-designate his legate for Constantinople.

It was at Bari, when about to sail for the East, that the news of the pope's death reached Desiderius.

Having obtained a safe-conduct from the Norman Count of Apulia, he had returned to his monastery and had been duly installed by Cardinal Humbert on Easter Day 1058.

A year later, Pope Nicholas II raised him to the cardinalate as Cardinal-Priest.

Desiderius has rebuilt the church and conventual buildings, perfected the products of the scriptorium and reestablished monastic discipline, so that there are two hundred monks in the monastery in his day.

On October 1, 1071, the new Basilica of Monte Cassino had been consecrated by Pope Alexander II.

Desiderius' reputation has brought gifts and exemptions to the abbey.

The money is spent on church ornaments, including a great golden altar front from Constantinople adorned with gems and enamels.

Peter the Deacon gives a list of some seventy books Desiderius had copied at Monte Cassino, including works of Saint Augustine, Saint Ambrose, Saint Bede, Saint Basil, Saint Jerome, Saint Gregory of Nazianzus and Cassian, the histories of Josephus, Paul Warnfrid, Jordanes and Saint Gregory of Tours, the Institutes and Novels of Justinian, the works of Terence, Virgil and Seneca, Cicero's De natura deorum, and Ovid's Fasti.

Desiderius had been appointed papal vicar for Campania, Apulia, Calabria and the Principality of Beneventum with special powers for the reform of monasteries.

Within two years of the consecration of the Cassinese Basilica, Alexander II had been succeeded by Hildebrand as Pope Gregory VII.

Desiderius has been able to exert the help of the Normans of southern Italy repeatedly in favor of the Holy See.

Already in 1059, he had persuaded Robert Guiscard and Richard of Capua to become vassals of St. Peter for their newly conquered territories: now Gregory VII immediately after his election sent for him to give an account of the state of Norman Italy and entrusts him with the negotiation of an interview with Robert Guiscard on August 2, 1073, at Benevento.

Pope Gregory VII withdraws all the censures of excommunication that he had pronounced, except those against the two chief offenders—Henry and Guibert—three days before his death in Salerno on May 25, 1085.

Desiderius, the great abbot of Monte Cassino, had in 1074 and 1075 acted as intermediary, probably as Gregory's agent, between the Norman princes, and even when the latter were at open war with the pope, they still maintained the best relations with Monte Cassino.

At the end of 1080, Desiderius had obtained Norman troops for Gregory.

In 1082, he had visited the Italian king and future Holy Roman Emperor Henry IV at Albano, while the troops of the Imperialist antipope were harassing the pope from Tivoli.

In 1083, the peace-loving abbot had joined Hugh of Cluny in an attempt to reconcile pope and emperor, and his proceedings seem to have aroused some suspicion in Gregory's entourage.

In 1084, when Rome was in Henry's hands and the pope besieged in Castel Sant'Angelo, Desiderius had announced the approach of Guiscard's army to both emperor and pope.

Though certainly a strong partisan of the Hildebrandine reforms, Desiderius belongs to the moderate party and cannot always see eye-to-eye with Pope Gregory VII in his most intransigent proceedings.

Yet when the latter lay dying at Salerno on May 25, 1085, the Abbot of Monte Cassino was one of those whom he had recommended to the cardinals of southern Italy as most fit to succeed him.

Under pressure from Prince Jordan I of Capua, to whom he has also rendered important service, Desiderius is elected Pope on May 24, 1086, taking the throne name of Victor III.

He is not at this time consecrated, owing to the presence of the Antipope Clement III in Rome.

Victor proceeds to Rome after celebrating Easter of 1087 in his monastery, and when the Normans have driven the soldiers of the Antipope Clement III (Guibert of Ravenna) out of St. Peter's, he is consecrated and enthroned.

Remaining only eight days in Rome, he returns to Monte Cassino, though with the help of Matilda and Jordan, he takes back the Vatican Hill.

Before May is out, he is once more in Rome in answer to a summons for the countess Matilda of Tuscany, whose troops hold the Leonine City and Trastevere, but when at the end of June the antipope once more gains possession of St. Peter's, Victor again withdraws at once to his Monte Cassino abbey.

A council or synod of some importance is held in August at Benevento, which renews the excommunication of the antipope Clement III and the condemnation of lay investiture, proclaims a kind of crusade against the Saracens in northern Africa and anathematizes Hugh of Lyons and Richard, Abbot of Marseilles.

When the council has lasted three days, Victor becomes seriously ill and retires to Monte Cassino to die.

Desiderius has himself carried into the chapter-house, issues various decrees for the benefit of the abbey, appoints with the consent of the monks the prior, Cardinal Oderisius, to succeed him in the abbacy, just as he himself had been appointed by Stephen IX, and proposes Odo of Ostia to the assembled cardinals and bishops as the next pope.

He dies on September 16, 1087 and is buried in the tomb he had prepared for himself in the abbey's chapter-house.

His successor is Pope Urban II.