Wikipedia
Gaumata or False Smerdis or Sphendadates (and various other names and aliases) appears in epigraphical and historiographical sources of classical antiquity as a late-6th century BCE Mede who usurped the Achaemenid throne by impersonating a member of the ruling family . The usurper is named 'Gaumata' in the Behistun inscription of Darius I, named '(false) Smerdis' in Herodotus' Histories, and is named 'Sphendadates' in the surviving fragments of Ctesias. Other sources have other names.
This story was immensely popular in antiquity and later in Hellenistic times, and many versions of the tale circulated around the rim of the western Mediterranean.
While the primary sources do not agree on the names and many other details, the three oldest surviving sources (Darius himself, Herodotus and Ctesias) all portray Gaumata/Pseudo-Smerdis/Sphendadates as an imposter who usurped the throne by posing as one of the sons of Cyrus (II), i.e. as one of the brothers of Cambyses. In Darius' trilingual Behistun inscription, the prince being impersonated is named 'Pirtiya' in Elamite, ' Bardiya' in Old Persian, and 'Barziya' in Akkadian. In the Histories, the prince and his imposter have the same name (Smerdis). For Ctesias, Sphendadates poses as 'Tanyoxarces'. Other Greek sources have various other names for the figure being impersonated, including 'Tanoxares', 'Mergis' and 'Mardos'.
The three oldest surviving sources also agree that Gaumata/Pseudo-Smerdis/Sphendadates is overthrown by Darius and others in a coup d'état, and that Darius then ascends the throne. Most sources (including Darius himself, Herodotus and Ctesias) have Darius as part of a group of seven conspirators. In Greek and Latin sources, Darius subsequently gains kingship by cheating in a contest.
The evaluation of the primary sources has been cause for much disagreement in modern scholarship. While there is agreement that Darius seized the throne through a coup d'état, there is dissent over whether Gaumata/Pseudo-Smerdis/Sphendadates was in fact an imposter, or whether Darius merely made him out to be one. The key argument against a fabrication is that there is no evidence for it. The key argument for a fabrication is that Darius had reason to invent the story since he had no particular rights to the throne.