Der Sarazene Der Heilige Krieg

Blurb

The Saracen is a two-part novel written by Robert Shea. The two separate portions, The Land of the Infidel and The Holy War are a continuous tale.
Basically ignored during its publication - and subsequently out of print, although still enjoying strong reviews and a cult following by those who have read it - the novel is the portrayal of an English-born man named David, who is captured as a very young child and sold into slavery to Baibars, a Mamluk officer. He becomes a devout believer in Islam and takes the Arabic form of his name and the surname of a convert, Daoud ibn Abdullah. He develops into a gifted warrior and assassin. He is sent to the Papal Court in Orvieto in the 13th century as a spy, in order to foil an alliance between the Christian West and the Mongolian descendants of Genghis Khan to exterminate the Muslim faith and capture the Holy Land.
Daoud was also trained by the Hashishyya, a heretical Islamic order. One of the many spellings of their name, Hashshashin, is where we derive the modern word "assassin". Shea spends considerable time discussing their techniques and philosophy, and it is a major theme of the book.