Darth Lokess's conspiracy

Several centuries after the founding of the reconstituted Sith Empire in 4,980 BBY, the Dark Lord of the Sith Darth Lokess proposed a plot to overthrow the Sith Emperor to her fellow members of the Empire's ruling Dark Council. The other Councilors agreed, but when they ascended the steps of the Imperial Citadel to confront the Emperor, he killed them all except for Lokess in a flash of light. Lokess was locked in the depths of the Citadel to be tortured until her death, and a new Dark Council was appointed.