They Used Dark Forces — Dennis Wheatley

It is 1943, World war II, and secret agent Gregory Sallust is parachuted into Nazi Germany. In the company of an ex-Bolshevik General named Stefan Kuporovitch, the two of them join forces with the widow of a German diplomat who is in contact with Allied Intelligence. It is through her that Gregory becomes unwillingly involved with a Black Magician and when, sixteen months later, they meet again, each decides to use occult forces in an attempt to destroy Hitler once and for all...