From the journal of Jedi Master Arlo Grennen during his investigation into the dark side's influence on Yavin 4:
“A few hundred years ago, the Jedi Order thought they could cleanse Yavin 4 of its dark influence. They were more aggressive then. I'm actually impressed. The Jedi bombarded the world from orbit with a destructive manifestation of light side power in the hopes it would free the moon from its dark embrace.
“The attacking Jedi destroyed much of the life on Yavin 4, which they later worked to restore to its previous state. They thought they'd won, that they'd removed the influence of the dark side. How wrong they were. The Massassi are still here. The dark side is still present. I would not be surprised if Exar Kun's spirit remains, waiting for someone‒a Jedi like myself, perhaps‒to find him….”