Vitiate is the most powerful Dark Sider ever.
To understand why this is a fact, we must analyse what he achieved on Nathema:
The ritual lasted ten days. Lord Vitiate orchestrated the sorcery and the planet Medriaas was consumed by the largest dark side nexus the galaxy would ever see.
- Star Wars: The Old Republic: Encyclopedia
The effects of this nexus are unprecedented, literally killing every living thing on the planet, even the Force, absorbing all:
When the ritual ended, Lord Vitiate emerged as the only survivor. The pain, energy, and suffering of every living entity on the planet fueled his power and would prolong his life for centuries. The lifeless planet of Medriaas became a void in the Force and was erased from history.
- Star Wars: The Old Republic: Encyclopedia
Vitiate himself absorbed said nexus and achieved its unprecedented power:
The Emperor is one thousand years old. In a dark side ritual, he drained the life of his home planet, to achieve immortality.
- Star Wars: The Essential Reader's Companion
Infact, a thousand years after the ritual, its effects are still existent within Vitiate himself:
Staring into the hollow darkness of the Emperor’s gaze, Scourge’s mind flashed back to Nathema, and he shivered at the memory.
- Star Wars: The Old Republic: Revan[/I]
Scourge cried out in anguish as the Emperor’s mind brushed against his, then he collapsed to the floor, shaking like a child. The touch lasted less than a second, but in that time he witnessed indescribable horrors that dwarfed anything the dark side could conjure even in his worst nightmares. And beneath the formless terrors lurked the unbearable Void, the pure emptiness of total annihilation.It was over as quickly as it had begun, the awful vision retreating into his subconscious like a repressed memory as Scourge picked himself up off the floor.
- Star Wars: The Old Republic: Revan
Vitiate as a product of the Ritual of Nathema became more powerful than any nexus ever seen. Another dark side nexus, is the vaunted Darth Sidious:
Eager to meet his captor, Luke learned that the dark side nexus he had sensed was none other than the cloned reincarnation of Emperor Palpatine himself.
- The New Essential Chronology
Darth Sidious is a very powerful dark side nexus, so much so that his energies tore the fabric of space:
The key to Luke's turning is the moment when he and Leia realize the Emperor is no longer defined by his physical form, but has become a chaotic nexus of dark energies that swell and burst open the fabric of space, tearing apart everything in the vicinity, human and machine.
- Dark Empire Endnotes
As impressive as this may be, Vitiate himself has gained such energies that they leave voids in life and the force, infact he continuously feeds more and more after the Ritual:
The Sith Empire's exile dragged on for centuries. While the military, navy, and Sith grew strong, the Emperor amassed unfathomable power and plotted a vengeful war against the Republic. He ruled from the privacy of his chambers in the Citadel, his life extended to virtual immortality by Sith sorcery.
- Star Wars: The Old Republic: Encyclopedia
Sources indicate that he is always feeding and draining others:
The temple became a nexus of dark side energy as the Emperor performed rituals within, drawing strength and knowledge from his captives.
- Star Wars: The Old Republic: Encyclopedia
Servants of the Hand share their Master's longevity, living untouched by age for centuries under his command. In return, the Emperor draws on his servant's strength in the Force and body to feed his ever-increasing power, leaving the servants withered and frail.
- Star Wars: The Old Republic: Encyclopedia
REVAN COULD FEEL THE EMPEROR FEEDING ON HIM, drawing on his power to sate his endless hunger.
- Star Wars: The Old Republic: Revan
Vitiate eventually achieved so much power that he didn't even require a physical form to reduce Ziost to the same fate as Nathema:
Global cataclysms are not unheard of. Whole worlds teeming with life have been rendered lifeless by meteorites, broken apart by instability in the planet's own core--even atomized by the destructive force of a supernova. But the eerie calm of a world stripped of life yet left otherwise intact is another matter altogether. Whispered rumors have persisted of planets snuffed out through intricate Sith rituals or by way of deadly, arcane machines--such as the device Revan sought to employ on Yavin 4--but Ziost represents a clear display of the corrosive power of the dark side of the Force taken to its extreme.
- Star Wars: The Old Republic: Rise of the Sith Emperor: Codex Entry "Death of a World"
He is infact repeatedly stated to be a literal embodiment of the dark side of the Force, with immeasurable power:
Enduring and merciless and quite possibly unkillable, Monoliths plainly illustrate the immeasurable power of their creator and are best avoided at all costs.
- Star Wars: The Old Republic: Rise of the Emperor Codex Entry "Monoliths"
The Emperor is more than a man - he is the living embodiment of the dark side.
- Star Wars: The Old Republic: Encyclopedia
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.
- Star Wars: The Old Republic: Encyclopedia
A being of unfathomable power and insatiable appetite, he transcended death multiple times, shedding his physical shells as they were discovered, defeated, and destroyed... only to return in another form.
- Star Wars: The Old Republic: Knights of the Eternal Throne: Codex Entry "The Fall of Valkorion"
In conclusion:
Tenebrae was the literal manifestation of the most powerful dark side nexus ever seen. A manifestation powerful enough to reduce planets to lifeless voids in the Force as he became ever more powerful, no matter his form. He was quite literally the living embodiment of the dark side of the Force, even more destructive than the likes of Malachor V or the legendary Darth Nihilus.