Wait here's the quote.
“He stayed there for a hundred years. When Marka Ragnos fell, Lord Vitiate did not join in the mad rush to claim his position. He was not part of the Great Hyperspace War against the Republic. When Naga Sadow and Ludo Kressh fought for leadership of the Sith, he took no side. But in the aftermath of the war—after our defeat at the hands of the Republic and our flight to escape the massacre of our people by the Jedi—he emerged from his solitude to call a great council of all the Sith Lords who remained. He invited them to his palace on Nathema, built on the site of his childhood home, the place where he had killed his adopted father and tortured his mother to death. He invited them to participate in a ritual to unlock the full potential of the dark side; he promised they would unleash power beyond anything they had ever witnessed or imagined.”“Didn’t they suspect a trap?”
“Perhaps.” Nyriss shrugged. “Some refused to answer his call. But many more came. After all, what could one man do against a hundred Sith Lords? Remember, he was not the Emperor back then. He was merely Lord Vitiate, ruler of a single planet of no particular importance. He hadn’t fought in any battles of note or achieved any great victories or conquests beyond his homeworld. He had the reputation of a scholar, not a warrior.
“And the Sith Lords were driven by fear. Many thought the Jedi would soon wipe them all out. They were desperate for anything they could use as a weapon against the servants of the light side. Lord Vitiate played upon these fears, convincing those who answered his call to set aside their suspicions of him and of one another to join in a single glorious cause.
“Once they arrived on Nathema, they quickly fell under Lord Vitiate’s control. He dominated their minds, crushed their resistance. He turned them into slaves to his will, forcing them to participate in the most complex ritual of Sith sorcery ever attempted. Calling on the dark side, Lord Vitiate devoured them. He fed on their power, absorbing it into himself, utterly obliterating all traces of his victims.
“But the ritual was not confined to the doomed Sith Lords. They were but the eye of the storm; the center of a vortex that spread across the entire planet. Every man, woman, and child on Nathema died that day. Every beast, bird, and fish; all the insects and plants; every living being touched by the Force was consumed. When the ritual ended, Nathema was no longer a world. It was a husk sucked dry. Lord Vitiate sacrificed millions, stealing their life force to make himself immortal. Their deaths also made him stronger than any Sith who had come before, and he ceased to be known as Lord Vitiate. On that day, the Emperor was truly born.”
So yeah he crushed their resistance and turned them into slaves before doing the ritual thing.