Second Sith theory

The Second Sith theory was a marginalized theory held by some members of the Jedi Order during the Separatist Crisis and later the Clone Wars. While not officially endorsed by the Jedi High Council, the theory postulated that the Zabrak dervish that killed Jedi Master Qui-Gon Jinn had been a Sith Lord. If this were true, then those who believed that the Rule of Two had been maintained among any Sith who remained hidden suggested there must be a second Sith—either a master or an apprentice. The theory was further advanced by Count Dooku's conversation with Obi-Wan Kenobi, where he implicated one Darth Sidious in manipulating the Galactic Senate.

Text and facts from the “Second Sith theory” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.