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Per Den of Geek, the Rebels series actually takes 14 years after Revenge of the Sith, so technically Reva and her inquisitor squad are the first of their kind to canonically appear in the Star Wars universe, timeline-wise!
The Inquisitors were in charge of hunting down surviving Jedi. If the Inquisitors failed to convince the surviving Jedi to turn to the Dark side and serve Emperor Palpatine, then they were deemed a threat that had to be exterminated then and there.
Source: https://www.distractify.com/p/moses-ingram-inquistor-reva-in-obi-wan-kenobi
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