You may be thinking of the Khazarian Jews. It has been stated that the King of Khazar converted to the Jewish faith and ordered his subjects to also do so. It is rumored that the Rothschilds actually came from Khazar.
There are arguments pro and con about this. It has also been stated that Ashkenazi Jews descended from the Khazars, but that is in dispute also.
I personally make no assertion one way or the other.
The Khazarian argument is used by anti-Semites, especially supporters of ‘Palestine’, to obviate any European Jewry claim to Israel with the added feature of invalidating Old Testament texts of the Babylonian Diaspora.
DNA studies of those with European Jewish ancestry have instead verified a chain of Semitic DNA reaching back to Biblical times.
The entire issue is fraught and best consigned to peripheral status, IMO, although many would probably be just as happy to disown the Rothschilds.