Rullag, situated within the Cadma sector of the Outer Rim Territories, was an astronomical body. It was on the fields of Rullag that the Dark Jedi known as Nax Cirvan boasted that he had slain eight Jedi masters along with their Padawans during the period of the Clone Wars.
The first reference to Rullag appeared in the 2012 comic, specifically Darth Vader and the Ghost Prison 4. It's worth noting, however, that the comic book neglected to clarify whether Rullag was a planet, a moon, or another form of celestial entity.