Originally posted by XSUPREMEXSKILLZ
The context of the dark council's death is unknown, they were unprepared for any sort of confrontation, and vitiate may very well have set a trap/utilized a ritual.
No!
During a violent tempest, Darth Lokess called a secret meeting of her eleven fellow Dark Council members. Under the cover of night, the Dark Council met and heard Darth Lokess describe a brilliant and daring plan to overthrow the Sith Emperor, end his relentless exile, and lead the Empire back to war against the Republic. An army of Sith under the command of Darth Lokess were stationed outside the meeting, ready to slay any Dark Council member who refused. But they all agreed to depose the Emperor.
It remains a mystery how the Emperor learned of his council's treachery. He allowed their plan to play out, but when they gathered to confront the Emperor, his punishment was swift and devastating. Eleven members of the Dark Council died in a sudden flash on the steps of the Citadel. The last member Darth Lokess, disappeared forever, though for some centuries some claimed her screams could be heard from the bowels of the Citadel.
A new council was immediately appointed to replace the traitors. The Emperor had made his point: his will was absolute, and none would oppose him. And though this was not the last time the council would defy the Emperor, his lesson would humble the Empire and its Sith leaders for centuries to come.
Source: (Star Wars: The Old Republic: Encyclopedia, Page 156)
1. This particular Dark Council prepared itself to eliminate Vitiate.
2. Vitiate also prepared himself to eliminate this particular Dark Council after he came to know about its treacherous intentions.
3. Vitiate unleashed a mysterious power to eliminate much of this Council. Lokess somehow survived but she was as good as dead nonetheless.
4. No ritual involved in this contest; this source have explicitly pointed out rituals based actions of Vitiate otherwise.
Rituals are typically performed to unleash dark side powers on mass destructive scale. Same powers can be unleashed without rituals but would be less potent in scope. Do you know that even Though Bomb can be unleashed singlehandedly?
Vitiate is an extraordinarily powerful Force-user by all accounts. He have controlled highest number of Force-users in galactic history.
Originally posted by XSUPREMEXSKILLZ
In a fair contest in which both are prepared to fight each other, Luke would know of vitiate's mind domination capability, and would create ample mental barriers against it (much like Revan taught to Meetra/Scourge). Plus, when is it shown that vitiate is immune to the effects of many force powers?
Ever seen any Force power harming him? He have been struck down by a lightsaber.
Also, their are some techniques which can bypass formal defenses of Force-users. Being a (super) Sith Sorcerer, Vitiate would be well-versed in undermining formal defenses of his opponents. This is why he have such an amazing combat record and have broken and killed so many individuals in the mythos. His downfall have circumstances attached to it.