The Nekghouls were a mutant strain of the Rakghoul Plague that originated on Taris, evolving over 3 centuries before their discovery by Republic Settlers during the time of SWTOR. While not much is known about them, the Nekghouls are unable to infect others with the Rakghoul Plague, and have gained both Force Sensitivity and high levels of intelligence and sentience compared to their regular Rakghoul counterparts that remain as rabid animals.

Rukil and the other Outcasts of Taris would find the Promised Land that Revan had aided them on their journey to discover before the Destruction of Taris at the hands of Darth Malak. Unfortunately after some years after reaching the Promised Lan, many of the Outcasts would be murdered by Nekghouls until the Outcasts were all driven to extinction.

It is completely unknown whether the first Rakghoul to mutate into a Nekghoul was a Jedi or Sith, as the Nekghouls have no recorded history however they have been able to innately reproduce to keep their new species alive, as well as constantly commune with both the Dark Side of the Force as well as understand and use complex technology such as Holocrons.
Unlike Rakghouls, the Nekghouls appear to travel in groups, wear clothes and operate weapons, as well as the ability to attempt to communicate and speak civilly with Humans, making them an incredibly powerful and dangerous species compared to the Flesh Raiders of Tython.

Long ago, the Nekghouls came across the Holocron of the Jedi Master Bay-Yon Siu, who attempted to teach the Nekghouls how to understand and responsibly use the Light Side of the Force, however it was to no avail as the Nekghouls seemed to be incapable of resisting the Dark Side that came more naturally to them.
This would not stop Jedi Master Sulan some years later from gathering a group of Nekghouls and attempting to personally redeem them to the Light. Master Sulan was only stopped when the Imperial Invasion of Taris occured during the middle of the Cold War and was confronted by the Sith Thana Vesh, as well as some Imperial Heroes (player character).

While Master Sulan perished, it is not known if he was killed by Thana Vesh, the Imperial Heroes or was ravaged on by his own Nekghoul followers as accounts are unclear.

Rakghouls evolve with incredible speed, but until recently, they had never shown signs of intelligence. Larger and far smarter than their rakghoul cousins, nekghouls may qualify as a new species in their own right–they appear unable to infect and transform their victims, but instead possess sensitivity to the Force and violent, untrained talent in its use.Although nekghouls are capable of speech, they have no known history or record of their origins. Are they the result of a Jedi or Sith being infected with the rakghoul virus? A mutation caused by radiation from Taris’s shattered reactors? Studies may have to wait–for now, merely surviving the presence of these horrific beings is difficult enough.

Bestiary Codex Entry: Nekghoul