Well, I suspect that M* survived his fall at the end of False Emperor.
And BW pretty much confirmed that R* used "Fold-Space" and retreated to safety at the end of the Foundry.
So not everyone we've "killed" is dead.
Anyway according to Datamining we've actually got an Operation between killing the Dread Masters and Terror from Beyond it's tentatively called
But yeah, a lot of the Dark Council is dead (play as a Republic JK and do your Corellia class quests + Corellia and Illum storylines and you kill A LOT of the Dark Council