The bit about Smerdis the Magus having no ears is found in Herodotus Book 3 Chapter 69:
Darius was himself fond of boasting about cutting off ears:
"Two revolts received special treatment, both at the time Darius happened to be in Media. The Sagartian rebel, Cicantakhma, was sent to him: 'I cut off both his nose and ears, and put out one eye, he was kept bound at my palace entrace, all the people saw him. Afterwards I impaled him at Arbela'. Darius is still more expansive about the fate reserved for Fravartis the Mede: 'I cut off his nose, ears, and tongue and plucked out an eye; he was chained under guard at the gate of my palace and everyone could see him there. Then I impaled him at Ecbatana.' ... The intensity of the punishment should not be surprising; it wa customary in the Assyrian period and in the Acheaemenid as well. Cutting off the nose and ears was the normal form of torture for rebels and usupers, the Greek authors would note."
Pierre Bryant, 'From Cyrus to Alexander', p 123.
Cutting off the nose was also a punishment in ancient Egypt.
All of the kings Darius supplanted were accused by him of lying about their Acheaemenid ancestry and therefore adjudged guilty by him of being rebels and usurpers. This of course is unrealistic. Darius was trying very hard to prevent the label of usurper being applied to himself.
Smerdis the Magus (Gaumata) and nine other subjugated kings are depicted on the Behistun relief of Darius. He claims to have vanquished all of them in 19 battles over a period of a single year. However, the sequence of the battles may be only generally chronological and it is not clear when that one-year interval began and ended. (Bardiya, Cambyses II, and Smerdis all died within about a year of one another.)
Hydarnes had engaged one of the "liar-kings", Fravartis the Mede, but as quoted above, Darius claimed to be the one that mutilated his face and impaled him. Fellow conspirator Intaphernes defeated another "liar-king" Arkha of Babylon, but Hydarnes was said to be the one who impaled him. The Behistun relief of Darius implies that Fravartis and Arkha were killed after Gaumata, but this clearly cannot have been "honest-to-Ahura Mazda true" if Hydarnes was one and the same as Cambyses II.
Hydarnes is not attested after the turbulent period after the death of Cambyses (522 BC) after which Darius came to power (521 BC), nor is Otanes, nor is Ardumanis, the three apparent aliases of Cambyses II/Tanuatamon. The final odyssey of Otanes is believed to have been an attack on Greek Samos in 520 BC, but the dating is suspect. According to Pierre Bryant, "We have no explicit mention of Hydarnes after 520", as well, although the names of Hydarnes and Otanes lived on through later princes.
The intense conflict involving Taharqa (Astyges/Thukra) and Tanuatamon (Cambyses) in Egypt with Assurbanipal (Smerdis) and Nabu-sharu-user (Nebuchadrezzar/Bardiya) of Mesopotamia created an opportunity for Mentuemhet (Hystaspes) and his sons Nes-Ptah II (Cyrus II) and Osorkon V (Darius) to seize the throne. They were the proverbial last men standing.
Darius, in his famous inscription, is taking credit ("by the grace of Ahura-Mazda") for mopping up the mess left behind by the epic feud of Taharqa, Tanuatamon, Assurbanipal, and Nabu-sharru-user/Nebuchadrezzar. Darius implicitly shares the credit with only two others, who stand behind him in the Behistun, but are not named there. Pierre Bryant concludes that they are the same two honored by Darius in his tomb, that is, Aspathines [our Cyrus II] and Gobyras [our Apries]. In fact, certain areas of the "mop up" more closely associated with the operations of Cyrus II, such as in Asia Minor, are not mentioned by Darius.
© Charles N. Pope, US Library of Congress. All rights reserved.