Complex sentence: The Reavers are mentioned several times during the course of the television show, a Reaver ship is seen in the pilot episode, and the episode Bushwacked shows someone turning into a Reaver.
Split into simpler sentences: The Reavers are mentioned several times during the course of the television show. A Reaver ship is seen in the pilot episode, and the episode "Bushwacked" shows someone becoming a Reaver.