From the journal of Jedi Master Arlo Grennen during his investigation into the dark side's influence on Yavin 4:
“Just as Naga Sadow trained his eventual replacement, so did Freedon Nadd's spirit find a powerful apprentice in Exar Kun. And just as Nadd killed his lord Sadow, Kun destroyed Nadd's spirit here on Yavin 4. These notable deaths likely served to further imbue the moon's temples with dark power, further fueling the barbaric Massassi‒as well as the succession of Dark Lords who would later reside here.
“I think Kun saw what Naga Sadow had accomplished here with Sith alchemy and architecture and decided he could do better. He forced the Massassi to build new temples, but this time with a focus on complementing and augmenting the dark side. Kun even went so far as to use various technologies to manipulate the Force in ways that I can't say I understand. I can't, but I would like to….”