Another tale of Yoda's past involves stopping a swath of destruction cut by Dark Jedi from Bpfassh. One of these dark siders had gotten as far as Dagobah, were Yoda bested him in direct confrontation. The Bpfasshi Jedi died, his dark Force absorbing into and twisting a nearby tree -- forming a dark side nexus. It is believed that Yoda chose Dagobah as his hiding place due to the dark side energies emanating from the tree. From a distance, Force-users like the Emperor would not detect Yoda since the master's bright light side presence would be canceled by the Bpfasshi dark side presence.
—Star Wars Databank
Years ago, during the Bpfasshi Dark Jedi insurrection, one of the fallen had escaped to Dagobah by commandeering a vessel piloted by smuggler Jorj Car'das. The Jedi Master Yoda tracked down the fugitive Jedi, and the two battled on the swamp planet. Yoda was victorious, and the dead Dark Jedi's hateful energies were absorbed into his surroundings.
A nearby tree which grew over a cave became a nexus, a concentration of the dark side. This domain of evil became a deciding factor in Yoda's choice of Dagobah as his refuge during the Empire's ascent. The dark energy of the tree, when viewed from a distance, cancelled out the light energy of Yoda, thus keeping him hidden for decades.
–Star Wars Databank
Fully in the grip of the Dark Side and quite mad, Bpfasshi Dark Jedi set themselves against Freedom's Sons and slaughtered everyone who get in their way. Soon opposed in battle by loyal Coruscant Jedi. The Bpfasshi fell one by one, until their leader ultimately perished in an epic Force Clash with Master Yoda.
—Star Wars: The New Essential Chronology
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"You presume limits to our double standards. There are none." - Vitidiots