The Monster-Slayer was a Sith of Sith Legend.
In the legend, The Monster-Slayer duels with rival Sith, The Dreamer. The duel ends with both dead, the two Sith overwhelmed by the vapors of the Pits of Paxxis.
According to Luke Skywalker, the legend was rooted in fact, and took place circa 3400 BBY.
At some point, Skywalker acquired a depiction of The Monster-Slayer's duel against The Dreamer, which he annotated sometime after the Emperor's death in 11 ABY along with a copy of The Jedi Path and Book of Sith.
Sources
- The Jedi Path: A Manual for Students of the Force (Art print with the 2014 re-release)
- Book of Sith: Secrets from the Dark Side (Art print with the 2014 re-release)