The duel between The Dreamer and The Monster-Slayer was a duel in Sith legend between two Sith: The Dreamer and The Monster-Slayer. The two fought for Sith supremacy, but both were overcome by vapors from the and died.
According to Luke Skywalker, the legend was rooted in fact, and took place circa 3400 BBY.
At some point, a depiction of the duel came into Skywalker's possession, which he annotated sometime after the Emperor's death in 11 ABY along with a copy of The Jedi Path and Book of Sith.