In 1081, the Norman Duke Robert Guiscard of Apulia prepared to invade the Byzantine Empire under the pretext of defending the succession of Constantine Doukas, who had been engaged to Robert's daughter Helena. Alexios I Komnenos was entrusted with an army and sent to defeat the Norman threat, but instead conspired with his relative John Doukas to seize the throne. Alexios raised a rebellion against Nikephoros and was able to quickly encircle Constantinople and besiege it, due to the lack of a defensive army. Nikephoros was unable to secure the support of either the Seljuk Turks or Nikephoros Melissenos, his traditional rivals, and therefore was forced to prepare for abdication. Nikephoros decided that his only choice was to abdicate in favor of Melissenos, who was nearby in Damalis in Anatolia, and sent messengers to him across the Bosphorus; however, these messengers were intercepted by George Palaiologos, a general of Alexios, who persuaded them to support Alexios. Alexios and his forces broke through the walls of Constantinople on 1 April 1081 and sacked the city. Nikephoros fled and sought sanctuary inside of the Hagia Sophia. Nikephoros was taken from there to the Monastery of Peribleptus, where he abdicated and became a monk. Nikephoros died later that year, on 10 December 1081.