Who is The Caliph from Eaters of the Dead: The Manuscript of Ibn Fadlan and what is their importance?

The Caliph is Ibn Fadlan's ruler, as well as the one who charges Ibn Fadlan with the task of traveling North to the King of the Saqaliba. Ibn Fadlan is not held high in the Caliph's esteem, but rather, is given his task as the result of a dalliance with the wife of one of the Caliph's friends..... a kind of punishment.