Very well done, but it isn't the same as Vitiate's ritual, the Ancient Sith techniques Kreia talked about are NOT referring to Nihilus, but the Sith Assassins themselves, they were techniques used so they could perfectly gouge their prey and kill them, but the Exile grew too powerful, too quickly for them.
Nihilus is a random case that randomly happened due to his proximity to the MSG's detonation on Malachor V, he was a larger wound than the Exile because he was much closer, the Exile only felt the shockwaves, something she was able to resist due to her innate ability to sever force connections.
Nihilus can only be weakened enough if you force him into using his hunger on deadened worlds or people, like the Exile, he ended up absorbing some of the Exile's death and it nearly killed him, the Exile finished off the job.
Nihilus would be something the Emperor would never see coming, just like the combined Jedi Order couldn't.
But regardless the writing of this is top quality.
Thankyou, but I would disagree. I based the whole scenario of Vitiate being aware of Nihilus's power on what Kreia said to the Exile (DS only):
“It is also possible to draw upon the strengths of others, to increase your own. It is similar to drawing upon the Force as Jedi do, but when it is touched by the power of the dark side… it is something else, something deadly. These Sith we face… they have learned how to do this. It is a technique that has been lost for some time, not seen in the days since the ancient Sith.
They can use it to consume other Force Sensitives and at the highest pinnacle of power, use it to consume anything that lives. The blind seer, her Master has harnessed this technique, and he is rapidly approaching the height of its power. I fear he may even rival some of the ancient Sith.”
Kreia is making it clear here, that Nihilus has harnessed the power used by the Ancient Sith and possibly know to Vitiate. Of course what Vitiate did on Nathema is not the same, i should have made that more clear, I think he is instead attempting to replicate it. Because the power is instinctive and therefore those who possess it are something of an anomaly as you said. Vitiate probably tried to perform the ritual himself but instead deliberately and therefore needed complex preparation to complete it. But in the end, I think he was aware of the ritual and therefore knew how to counter it.
As for not seeing him coming, no he wouldn't, which is the reason Vitiate was ultimately defeated because despite being right infront of him he could still not 'see' him. But Vitiate could hear the echoes he created, the deaths of millions can be felt across the galaxy by force sensitives, like Obi-Wan did with Aldreean. So Vitiate would have sensed the echoes and sent Scourge to investigate. The reason Vitiate helped Scourge find him, and I didn't actually mention this, is the patterns he saw in the dead planets which showed the path Nihilus was taking, like following bread crumbs.
Hope that explains it a bit more, a thanks for reading it