I know I'm new to the faction and all but I may have an idea or two
-An old dark side entity thousands of years old is released from an tomb that looks sith in origin,by a few Jedi Shadows tasked with finding,or destroying old sith artifacts they are drawn to the dark precense by hearing a voice calling to them. One of them accidently releases a barrier of some sorts,and a sarcophagus lies in front of them radiating with the dark side. They cut it open with their lightsabers,and all they see is nothing. They hear loud sadistic cackling. Than a shadow figure manifests itself,and in their horror the Jedi Shadows try to fight back but are overpowered with ease by the entity,and are possessed by the spirit after being freed sends the shadows back to infiltrate the SS. They are sent to kill their masters,and friends alike as he plans to regain his strength and destroy both jedi and sith,by trying to ressurect an ancient army of his through sith alchemy making them an undead army,and we have to destroy him before it's too late.
Sorry if it's long I'm just trying to throw ideas out there.