Perhaps immune is the wrong word. They devour the Force.
The great Sith Tulak Hord defeated him in battle, and he joined his service, feeding upon thousands of Jedi in the course of that ancient war. They developed a bond, almost like friendship.
Tulak put him in stasis in a tomb on Korriban, which is where I found him.
I defeated him when I freed him, and he chose to serve me. I suspect, if I do not earn his respect, he will some day attempt to feed on me.