Date of birth unknown; reigned 955-64.
The younger Alberic, after the downfall of his mother, Marozia (932), was absolute ruler at Rome. Before his death he administered an oath (954) to the Roman nobles in St. Peter's, that on the next vacancy of the papal chair his only son, Octavius, should be elected pope. After the death of the reigning pontiff, Agapetus II, Octavius, then eighteen years of age, was actually chosen his successor on 16 December, 955, and took the name of John. The temporal and spiritual authority in Rome were thus again united in one person — a coarse, immoral man, whose life was such that the Lateran was spoken of as a brothel, and the moral corruption in Rome became the subject of general odium. War and the chase were more congenial to this pope than church government. He was defeated in the war against Duke Pandulf of Capua, and at the same time the Ecclesiastical States were occupied by Berengarius, King of Italy, and his son Adalbert. In this dilemma the pope had recourse to the German king, Otto I, who then appeared in Italy at the head of a powerful army. Berengarius, however, did not risk an encounter, but retired to the fortified castles. On 31 January, 962, Otto reached Rome. He took an oath to recognize John as pope and ruler of Rome; to issue no decrees without the pope's consent; and, in case of his delivering the command in Italy to any one else, to exact from such person an oath to defend to the utmost of his ability the pope and the patrimony of St. Peter. The pope on his part swore to keep faith with Otto and to conclude no alliance with Berengarius and Adalbert. On 2 February, 962, Otto was solemnly crowned emperor by the pope. On the twelfth a Roman synod took place, at which John, at Otto's desire, founded the Archbishopric of Magdeburg and the Bishopric of Morseburg, bestowed the pallium on the Archbishops of Salzburg and Trier, and confirmed the appointment of Rother as Bishop of Verona. The next day, the emperor issued a decree, the famous Diploma Ottonianum, in which he confirmed the Roman Church in its possessions, particularly those granted by Pepin and Charlemagne, and provided at the same time that in future the popes should be elected in canonical form, though their consecration was to take place only after the necessary pledges had been given to the emperor or his ambassadors. The authenticity of the contents of this much-discussed document is certain, even should the extant document be only a duplicate of the original (Sickel, "Das Privilegium Ottos I, für die römische Kirche", Innsbruck, 1883). On 14 February the emperor marched out of Rome with his army to resume the war against Berengarius and Adalbert. The pope now quickly changed his mind, while Otto on his part urged the imperial authority to excessive limits. John began secret negotiations with Adalbert, son of Berengarius, and sent envoys with letters to Hungary and to Constantinople for the purpose of inciting a war against Otto. They were, however, seized by the imperial soldiers, and the emperor thus learned of the pope's treachery. John now sent an embassy to Otto to propitiate the latter, and at the same time to explain the pope's grievance, which was that the emperor had received for himself the oath of allegiance from those cities of the Ecclesiastical States, which he had reconquered from Berengarius. Otto sent an embassy to refute this accusation. At the same time Adalbert came in person to Rome, and was ceremoniously received by the pope. The faction of the Roman nobles which sympathized with the emperor now broke into revolt against John. Otto appeared for the second time in Rome (2 November, 963), while John and Adalbert fled to Tivoli. In the emperor's entourage was Liutprand, Bishop of Cremona, who thus describes the occurrences as an eyewitness. Otto now probably renewed and extended the settlement formerly effected, by obtaining from the nobles a promise on oath not to elect or consecrate a pope without the consent of the emperor.
On 6 November a synod composed of fifty Italian and German bishops was convened in St. Peter's; John was accused of sacrilege, simony, perjury, murder, adultery, and incest, and was summoned in writing to defend himself. Refusing to recognize the synod, John pronounced sentence of excommunication (ferendæ sententia) against all participators in the assembly, should they elect in his stead another pope. The emperor now came forward to accuse John of having broken the agreement ratified by oath, betrayed him, and called in Adalbert. With the imperial consent the synod deposed John on 4 December, and elected to replace him the protoscriniarius Leo, yet a layman. The latter received all the orders uncanonically without the proper intervals (interstitia), and was crowned pope as Leo VIII. This proceeding was aginst the canons of the Church, and the enthroning of Leo was almost universally regarded as invalid. Most of the imperial troops now departing from Rome, John's adherents rose against the emperor, but were suppressed on 3 January, 964, with bloodshed. Nevertheless, at Leo's request, Otto released the hundred hostages whom he had called for, and marched from Rome to meet Adalbert in the field. A new insurrection broke out in the city against the imperial party; Leo VIII fled, while John XII re-entered Rome, and took bloody vengeance on the leaders of the opposite party. Cardinal-Deacon John had his right hand struck off, Bishop Otgar of Speyer was scourged, a high palatine official lost nose and ears. On 26 February, 964, John held a synod in St. Peter's in which the decrees of the synod of 6 November were repealed; Leo VIII and all who had elected him were excommunicated; his ordination was pronounced invalid; and Bishop Sico of Ostia, who had consecrated him, was deprived forever of his dignities. The emperor, left free to act after his defeat of Berengarius, was preparing to re-enter Rome, when the pope's death changed the situation. John died on 14 May, 964, eight days after he had been, according to rumour, stricken by paralysis in the act of adultery. Luitprand relates that on that occasion the devil dealt him a blow on the temple in consequence of which he died.
Liber Pontif., ed. Duchesne, II, 24609; Jaffé, Regesta Rom. Pont., I (2nd ed.), 463 sq.; Liutprand, De rebus gestis Ottonis, ed. Dümmler, Opp., 124-36; Hergenröther-Kirsch, Kirchengesch., II (4th ed.), 201-7; Langen, Gesch. der römischen Kirche, II, 336-51; Reumont, Gesch. der Stadt Rom, II, 237 sqq.; Hefele, Konziliengesch., IV (2nd ed.,), 605 sqq.; Dümmler, Otto der Gross, V, 313 sqq.