Force Drain:
Vitiate was a master of Force Drain and associated talents, capable of draining enormous number of individuals simultaneously.
Vitiate created a "gigantic reservoir of energy" for himself from the ritual he orchestrated on Medriaas, leaving this planet a void in the universe. Vitiate consumed this reservoir of energy with his dark sorcery (Force Drain involved), making himself more powerful then he ever had been, prolong his life for centuries, prevent decay of his original body and even share his immense power with other Force-users (minions) on galactic scale to increase his reach accordingly.
Reminder:
From Star Wars: The Old Republic: EncyclopediaThe pain, energy, and suffering of every living entity on the planet fueled his power and would prolong his life for centuries.
After this power progression, Vitiate was capable of pulling off Nathema like dark sorcery "without external aid" because his capacity as a practitioner of the Force had vastly increased in the aftermath:
From Star Wars: The Old Republic: EncyclopediaLord Vitiate takes command of the Sith Empire, now in shambles following the Great Hyperspace War. He executes the Sith Council and consumes the life force of thousands of Sith Lords in a terrifying ritual that extends his life and vastly increases his capacity as a practitioner of the Force.
Revan figured out this fact as well:
From Star Wars: The Old Republic: Revan"Nathema was just the beginning," Scourge agreed. "He will destroy world after world, his power and madness growing in concert until he alone is left, Emperor over an empty and lifeless galaxy."
Meetra stared at the two in horror.
"You've been to Nathema," Scourge said. "You felt the Void. You know what the Emperor is capable of."
"She understands," Revan said, reading her expression more accurately than Scourge. "That's not it."
"He's quarantined Dromund Kaas," Meetra said, trying to lead them to the same conclusion. "What if he’s preparing to do the same thing here that he did on Nathema?”
Scourge hadn't considered that possibility, and it chilled him to his core.
"Is that possible?" he asked. "Nyriss told me the ritual on Nathema took days, if not weeks. And the Emperor had to trick hundreds of other powerful Sith into working with him so he could draw on their power."
"He's stronger now," Revan said. "But even if it's possible, I don't think he'll go that far. At least not yet. He is too patient, too careful. Dromund Kaas is the heart of his Empire and the seat of his power. He has too many valuable resources here to throw it all away. But once he is ready, there will be nothing left to stop him from launching his invasion of the Republic."
NOTE: For additional information, please recheck the Sense section.
Force blasts:
Vitiate could reshape his crackling bolts or offensive telepathic applications in to concentrated blasts of energies which he could unleash on his opponents to overwhelm them. Vitiate unleashed this power on a Jedi Strike Team of some of the most powerful Jedi of the Order to break it, twisting them all to the dark side.
Telepathic abilities:
Vitiate possessed unprecedented command of telepathic abilities, utilizing such powers for offensive purposes as well. In other words, Vitiate's telepathic abilities encompassed all aspects of the Force: Control, Sense and Alter.
Star Wars: Tales of the Jedi Companion confirms that powers such as Control Mind require combination of Contol, Sense and Alter abilities to work.
Vitiate could affect and control minds of his targets to persuade them to do his bidding against their will (reducing them to puppets), inflict pain on them, disorient them by dampening their senses, probe them and even destroy their minds. Vitiate demonstrated the power to break even powerful Force-users, an impossible task for most Force-users.
Vitiate have gone as far as to break multiple powerful Force-users simultaneously. To give you an idea of how difficult is this task: it took combined might of Mace Windu, Obi-Wan and Anakin Skywalker to compel a normal but strong-willed individual (Cad Bane) to reveal the truth to them during an interrogation process. In contrast, Vitiate's telepathic abilities are on a level which is difficult to comprehend; he would single-handedly broken the aforementioned Jedi instead and not just Cad Bane.
From Star Wars: The Old Republic: EncyclopediaOver 300 years ago, the great Jedi heroes Revan and Malak stumbled upon long-hidden Sith Empire's capital of Dromund Kaas, and its ruler - a mysterious, almost godlike avatar of the dark side. They argued briefly over whether to alert the Republic and Jedi Council, but Revan was already too consumed by arrogance and anger to consider the possibility of defeat. By the time Revan and Malak approached the Emperor in his throne room, they were already at the precipice of the dark side. It took only a fraction of the Emperor's loathsome power to complete their fall. The Jedi succumbed utterly to the Sith leader's domination and returned to the Republic to spark a new conflict: the Jedi Civil War.
From Star Wars: The Old Republic: EncylopediaThe Jedi dispatched scouts and spies to learn as much as they could about this new enemy and its enigmatic leader. However, the Sith Emperor was a master of subterfuge and misdirection, continually thwarting all efforts to uncover his dark secrets.
Most Jedi who ventured into Imperial space vanished without a trace, but a few managed to send back disturbing messages to their Masters. These fragmented communiques showed the Emperor's Jedi pursuers descending into fear, madness, and evil when faced with his power. It quickly became clear the Sith Emperor was more then a brilliant military strategist and cunning political leader. He was a living embodiment of the dark side of the Force who delighted in destroying the minds and spirits of those Jedi who came too close to him.
From Star Wars: The Old Republic: Game Codex Entry (The Emperor's Fallen Jedi)The Sith Emperor has mastered the dark side’s power to become the most dominating Force-user the galaxy has ever seen. His corrupting influence is so complete that none can stand in his presence without succumbing to fear, anger and hatred. The Emperor can wither and ruin even the strongest Jedi’s connection to the light side. Jedi Master Tol Braga’s strike team was not the first group to succumb to the Sith leader’s oppressive influence. Hundreds of years ago, the Jedi Revan and Malak discovered Dromund Kaas and confronted the Emperor. They fell to the dark side and returned to Republic space as Sith Lords. Since then, dozens more Jedi have followed the same path into evil.
Sever Force:
Vitiate possessed the capability to sever the connection of Force-users from the Force itself to make then vulnerable like he did to Lord Dramath:
From Star Wars: The Old Republic: RevanLord Dramath intended to judge the child's power to determine if he was worthy of serving the Sith Lord, or if he should simply be executed. But Tenebrae had no intention of serving—or of dying. When they met face-to-face, Tenebrae proved the stronger. Only ten years old, he stripped his father of his power and his mind. Lord Dramath spent his last moments weeping in terror, gazing up into the black eyes of his son.
Vitiate performed the aforementioned feat when he was just 10 years old. Once can only imagine how proficient he would have become with this talent at the height of his power.
Mysterious powers:
Vitiate, being an absolute master of dark arts, was capable of summoning powers of extremely lethal proportions to destroy overwhelming odds.
Here is an example of Vitiate utilizing his dark sorcery in combat situations to overcome overwhelming odds:
From Star Wars: The Old Republic: EncyclopediaDuring 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.
Vitiate may have known lot of powers but content featuring him is limited at the moment so full extent of his capabilities are unknown accordingly as well.
Aptly put here:
From Star Wars: The Old Republic: EncyclopediaIn his relentless pursuit of immortality, the Emperor explored the most sinister, uncharted depths of the dark side.