Byss, the Emperor's private throneworld and the home to the majority of his dark side experiments. It is interesting to note that the planet's dark side aura was not due to years of cumulative encounters with a massive vergence in the Force or centuries of dark side adepts visiting, but actually Palpatine managed to corrupt the entire thing beyond repair (while all sources indicate that he spent the majority of his time secluded on Coruscant). The energies were so potent that the entire population became little more than zombified slaves, adhering to Imperial will.