Specters of "Sith Warriors" existed on the Sith homeworld of Moraband. These wraith-like apparitions were conjured by Force Priestesses as well as by the power of the dark side of the Force, in order to test if Jedi Master Yoda was ready to begin training to preserve his identity after death.
Yoda is surrounded by seventeen Sith Warrior specters.
When Jedi Master Yoda traveled to Moraband, he was confronted by seventeen of these specters, which were created by the Force Priestesses and the power of the dark side of the Force as part of a larger test to see if Yoda was to train to retain his identify after death. The apparitions encircled the Jedi Master, mocking him for his quest. After promising to betray his movements to the reigning Dark Lord of the Sith, the specters slammed against him, leaving repercussions in the Force which ended up warning Darth Sidious of Yoda's location.
The Sith Warriors first appeared in the Star Wars: The Clone Wars television series' sixth season. They were voiced by Corey Burton and Dee Bradley Baker.