The Sith wyrm, a gigantic entity, dwelled in expansive underground dens on Yavin's fourth moon.
Initially, the Sith wyrm began as an exogorth, but it underwent a transformation via Sith alchemy at the hands of Naga Sadow, the Dark Lord of the Sith, who intended it to serve as a guardian.
The Massassi of old revered the creature as a deity, that is, until Exar Kun slew it a thousand years afterward. Kun's motivation was to gain favor with Freedon Nadd's spirit. Unbeknownst to both Zythmnr, the Massassi shaman, and Exar Kun, Nadd was orchestrating this confrontation to control Kun. Kun could only triumph by harnessing the strength of a Sith amulet and fully embracing the dark side of the Force. It was subsequently discovered that the creature had started as a young exogorth that clung to Sadow's warship. Unable to control the being and finding no other purpose for it, Sadow chose to transmute it into the prototype Sith wyrm.
The fact that the Jedi subsequently pursued Sith wyrms during the Great Hunt indicates that this creature was not unique.