Complex sentence: They inform him that the Nazis, in their quest for occult power, are searching for his old mentor, Abner Ravenwood, who is the leading expert on the ancient Egyptian city of Tanis and possesses the headpiece of the Staff of Ra.
Split into simpler sentences: They inform him that the Nazis, who are obsessed with the occult, are searching for his old mentor Abner Ravenwood. The Nazis know that Ravenwood is the leading expert on the ancient Egyptian city of Tanis, and that he possesses the headpiece of the Staff of Ra.