In The Empire Strikes Back Luke has a run-in with the Dark Side tree where he confronts his fears and has a vision of Darth Vader.
In the Timothy Zahn’s Thrawn trilogy it’s explained that Yoda killed a Dark Jedi during the Clone Wars on that spot, and his evil infected the area making it strong in the Dark Side. We see something similar in Dark Force Rising–Leia experiences an emotional blast of the Dark Side in Endor’s orbit as her ship passes through the spot where the Emperor died. The implication–Dark Jedi leave traces of their evil long after they’ve gone.
I was hoping to see this fight between Yoda and a Dark Jedi in Revenge of the Sith. The hitch is, this backstory on the Dark Side tree doesn’t line up with Sith. If there are two Dark Jedi at any given time, and we know where each of these Dark Jedi are at any given moment in the films, there’s no “spare” Dark Jedi, strong in the Dark Side of the Force, to die at Yoda’s hands and infect the tree on Dagobah with the Dark Side.
This assumes, of course, that a Dark Jedi and Sith are synonymous. They don’t have to be–there could be Jedi who fall to the Dark Side who don’t become Sith. But the way the Republic Era is portrayed on film and in books Jedi just don’t fall to the Dark Side. Anakin’s fall to the Dark Side was portrayed as a singular event, and the novels note that Jedi who can’t function within the Order, like Count Dooku, are considered lost, not dark.
So, who did Yoda kill on Dagobah?