From A New Look at Sokullu Mehmed Pasha, published at Miskatonic University.
The consensus of historians on the assassination of Grand Vizier Sokullu Mehmed Pasha on October 11, 1579 AD (or 20 Sha'ban 987 AH in the Islamic reckoning) is fairly well-known in our field. The Ottoman Sultan Murad III, alienated from the vizier who had served his father and grandfather so long and ably by his mother Narbanu Sultan and Venetian-born wife Safiye Sultan, took steps to reduce the vizier's influence on government. The vizier's allies were sent to faraway positions or assassinated. Ultimately, a mentally-unstable dervish talked his way into the vizier's office and stabbed him. This kind of intrigue was fairly common in the Ottoman Empire, especially during the period known as the Sultanate of Women.
However, some recent discoveries by Miskatonic University researchers of documents thought lost forever during the civil unrest that wracked Constantinople when the Janissaries were suppressed has shed new light on the circumstances of the vizier's assassination and an incident that took place in 1571.
These documents paint a far more sinister picture of the vizier. They include accusations of dealings with agents of Safavid Persia, with whom the vizier had counseled peace as opposed to the usual border wars, and even black magic. The documents accuse the vizier of, under the influence of an agent of Persian Shah Tahmasp I, acquiring a book of black magic from an Armenian merchant who had visited the long-vacant shrine of a corrupted Sufi order that had been destroyed by Turkish nomads not long before. The use of this book resulted in an incident in Constantinople that killed dozens of Ottoman soldiers, destroyed one war galley and forced the scuttling of a second, and caused significant damage to the Bayezit II mosque.
These accusations against Sokullu are not new, but have been long dismissed as the slanders from his political enemies. However, the mosque was damaged somehow, necessitating repairs by the famed Ottoman architect Mimar Sinan in 1573 and 1574. Furthermore, it is often said that converts make the best zealots. Safiye Sultan was a Catholic before she became a Muslim, while the most recent evidence suggests Narbanu was an Orthodox Greek from Corfu before her conversion. If Sokullu was involved in the dark arts, or was widely believed so, this could have provoked the ire of the Imperial women. They would not wish one so tainted to continue virtually ruling the Ottoman Empire in place of their son and husband. And the dervish orders might be willing to provide an assassin to dispose of the vizier, especially given his (tangential) connection to a Sufi order that had become warped by dark forces.
Of course, this is all just speculation. The documents describe how the soldiers killed in the incident were buried in a mass grave outside Constantinople that was given special attention by Muslim imams, Orthodox Christian priests, and even a Jewish rabbi, while the materials used by Sokullu in the incident were confiscated, burned, and abandoned in Persia. Should this mass grave or the dumping site be found, it would lend credence to the incident described in the documents.
So just why was the Grand Vizier assassinated, and is the author's theory about dark powers manifesting in Constantinople actually true? Read "The Beast of the Bosporus" either as a standalone short story or as part of the collection Flashing Steel Flashing Fire.