The Greekling lied about having a map to the realm of the Universal Religion and invented all the odd creatures that Baudolino wrote down in the letter of Prester Johannes. He then made a copy of that letter, an early example of copy right infringement (just kidding!) and disappeared. What he did with the copy of the letter greatly confused the Emperor and Baudolino while they were in Venice to make peace with the Pope. In the book, Zosimos is a plain villain, with not much of a physical description. In the movie though I will make him a good looking charlatan, fun in spite of everything; especially when Baudolino finds him, in Constantinople, in the middle of a necromancy session. The treatment in progress is the first movie adaptation in the timeline of the trilogy.