The Korriban Massacre was an event two years into the Chiss-Imperial War, resulting in the Jedi Order being crippled for the opening days of the Deity Crisis. It would eventually result in an eventual shift back to the original Masters Council, led by Luke Skywalker once more.
The Jedi Council greeted a dark visitor requesting training, judging whether he would be inducted into their Order. The man tugged down his hood, and the Jedi Masters were surprised to recognise a Sith in their midst, evident by his crimson blade. The man, calling himself 'Korriban' to a Knight who viewed the immediate aftermath - before they, too, were killed - proceeded to slaughter the Jedi Council, one and all, without hardly any scratch.
Korriban subsequently departed Coruscant, and the Deity Crisis began.