The Dashade, a force resistant reptilians alien race. Most were wiped out when their home world was destroyed. Many of them served as assassins, including under the command of Sith such as Tulak Hord. There are even rumors that some may have worked as combat instructors for the Jedi but I do not have duracrete proof of this.
Many of their race referred to a Mistress Death and their service to her. I do not know if this being exists now or has ever existed. But I theorize that this is where their force resistance could potentially come from. Perhaps she is the one who consumes and destroys those who cannot find life after death. The Dashade ability to consume the life essence of force sensitives could be a manifestation of this.
Sadly, without many of their race around to speak with directly, and their home world destroyed, there is not much that can be found or gained to help shed light on the matter.
It is possible she exists. Early Jedi and Sith became infatuated with becoming one with the force after physical death and immortality, respectively. Many still do, but, I wonder, if those early ones, knew of an aspect of the force and of life and death being incarnate and wishes to avoid her.
