There was no thirteenth tribe of Israel in the Bible. This is a modern antisemitic conspiracy theory with no support among scholars.
To illustrate, the following is taken from An Antisemitic Conspiracy Theory is Being Shared on Telegram to Justify Russia’s Invasion of Ukraine 5th May 2022 By Lea Gerster for the Institute for Strategic Dialogue PO Box 75769, London, SW1P 9ER
Conspiracy theories about the Khazars
Conspiracy theories about the Khazars, a semi-nomadic Turkic people who established a major empire (Khazaria) between Eastern Europe and Western Asia in the Middle Ages, have been spread in conspiracist, right-wing extremist, and Islamist communities online prior to the current conflict. However, given that Khazaria spanned modern-day, south-eastern Russia, southern Ukraine, Crimea and Kazakhstan, they have acquired distinct geographic relevance amidst the war in Ukraine.
The Khazars became a popular talking point among conspiracy theorists due to the claim that the Khazar people, or at least their elites, converted to Judaism en masse during the 8th century. The lack of reliable, contemporary source research on the Khazars make it difficult to determine how true this claim is. Khazaria was forgotten for centuries, until it was rediscovered among early Zionists in the 19th century, who were interested in what they saw as a potentially historic Jewish state. While there do appear to have been conversions to Judaism among the Khazars, the extent of the conversion to Judaism amongst the wider populace remains unclear.
Much of the contemporary interest in the Khazars by conspiracy theorists can be linked to “The Thirteenth Tribe”, a book written by Hungarian-British author Arthur Koestler in which he presents the highly contested thesis that Ashkenazi Jews are descendants of converted Khazars rather than of ancient Israelites. Koestler was aware that his theory could be exploited to delegitimise the State of Israel, but stated his hope that it would remove racial grounds for discrimination against the Jewish diaspora in Eastern Europe.
The use of Koestler’s theory by antisemites
Observation of German-language far-right and conspiracy Telegram channels in the present day would suggest that Koestler’s theory has been instrumentalised to promote antisemitism. In these channels, the terms “Khazars” and “Khazarian Mafia” have been used to describe Jewish people similar to how “Rothschild” and “Zionist” are used, ostensibly to soften antisemitic statements. More insidiously, the claim that Ashkenazi are not descendants of Israelites, but Khazars, allows antisemites plausible deniability by claiming that they only oppose “fake Jews” and “impostors” (as observed in Image 2).
Conspiracist and far-right actors online have also made direct reference to Koestler’s “The Thirteenth Tribe”. A convicted Holocaust denier from Germany referenced Koestler’s theory in a Bitchute video (Image 1), in which he speculated about a Jewish plan to build a “Heavenly Jerusalem” in Ukraine since most Jews were supposedly Khazars.
It’s important to note however that Koestler himself was not an anti-semite, nor was the author of the source of much of his arguments, Raphael Patai The Myth of a Jewish Race (1975), who like Koestler was Hungarian, but unlike Koestler was Jewish.