The Abyssinian Proof

Constantinople, May 1453.  In the dying days of the Byzantine Empire, as 7000 armed men prepare to defend the city against the might of the Ottoman Turks, Isaak and his family are entrusted with a purple velvet bundle.  Inside is a silver reliquary carved with the figure of a weeping angel and an inscription 'Behold the Proof of Chora, Container of the Uncontainable'.  It holds proof of God's existence.

In Istanbul in 1887: the local magistrate, Kamil Pasha, is plagued by thefts of antiquities from mosques and churches that, within days, appear for sale in Europe.  In addition, there has been a series of murders in which the bodies bear the mark of angel's wings.  Could this be connected to the rumours of an ancient cult?