We are aware of an issue with the Sith Raid that can cause Sion or Nihilus to become invincible if they are at very low health. We believe we have identified a fix this evening and are working to get it deployed as soon as we can. This fix is not likely tonight so please hold off on completing the Sith Raid if possible.
From our initial investigation, it appears that if a player hits this issue and then posts their score, it can lock the raid for the rest of the Guild if there are no battles by other guild members in progress. Anyone already battling should not be affected and can finish their run.
We will share more information as soon as we can. Thank you for your patience.