This morning, as with some kind of
six degrees of separation, I located the source for DariusGÇÖ single-handled combat. While searching for information on Waldemar HeckelGÇÖs last Alexander symposium (I want to know when the book is to be published) I came across HeckelGÇÖs original review of Maxwell OGÇÖBrienGÇÖs book. In it he tells how OGÇÖBrien relates and documents Dareios IIIGÇÖs victory over a Kadousian champion. I pulled
Alexander the Great: The Invisible Enemy off the shelf; checked the index; checked the page; checked the reference; checked the online
Diorodorus Book 17 and here we are:
Diordorus 17.6.1-3
[1] Dareius's selection for the throne was based on his known bravery, in which quality he far surpassed the other Persians. Once when King Artaxerxes1 was campaigning against the Cadusians, one of them with a wide reputation for strength and courage challenged a volunteer among the Persians to fight in single combat with him. No other dared accept, but Dareius alone entered the contest and slew the challenger, being honoured in consequence by the king with rich gifts, while among the Persians he was conceded the first place in prowess. [2] It was because of this prowess that he was thought worthy to take over the kingship. This happened about the same time as Philip died and Alexander became king. [3] Such was the man whom fate had selected to be the antagonist of Alexander's genius, and they opposed one another in many and great struggles for the supremacy. These our detailed narrative will describe in each case. And we may now proceed with our story.
(Btw, OGÇÖBrienGÇÖs book is now $109.67 new from Amazon! Yikes! Thankfully there are still a few used copies available starting at $14 GÇô and a few more inexpensive copies can be found via the
Addall search engine. And, no, IGÇÖm not being paid to promote this book!)

My edit, later in the day: ItGÇÖs just been pointed out to me that new
paperback copies are available at a price of $37.95! ThatGÇÖs a relief!
Best regards,