A book by Michael Moorcock. Like his other novels, takes part in the Eternal Champion multiverse.
a brief summary of the beginning part
von Bek has been a mercenary and fighting in wars for a long time. (The books setting is right around when guns were starting to get the upper hand on long, pointy sticks.).
He suddenly finds a castle in the middle of nowhere, and promptly falls in love with the beautiful lady therein.
Then, enter Satan. Satan tells von Bek that he has a proposition. Satan wants to get back into the good graces of God, and believes that it would help to recover the Holy Grail and give it to God. Satan points out that he already has dibs on von Bek's sin-ridden soul. If von Bek finds the grail, not only will he give up his rights on his soul, but also release the soul of the aforementioned beautiful lady...
then the rest of the adventure begins...
As I recall, not all von Beks are incarnations of the Eternal Champion (like Elric, Erëkose, Corum, etc.). Instead, the von Beks have a special affinity for the Holy Grail, which I think is an instrument of Cosmic Balance in the Multiverse.
Like many of Michael Moorcock's novels, the protagonist has a special weapon. In this book, it is the Sword of Paracelsus.