Originally posted by Dolos
Wrong.Malgus experienced it after killing a Grand Master of the Jedi Order. Plagueis felt a surge of power a little while after killing Tenebrous. Sidious felt a larger surge of power immediately after killing Plagueis.
Unity with the Force can entail either the light side or dark side. Pure good or pure evil.
The Sith are altogether stronger, they get stronger faster, they get stronger more easily, and ultimately two [Sith] are just as formidable as an entire order [Jedi].
The Force itself has no light nor dark. Lightside and darkside is determined by emotions of a user.
However, the Force has a will. Jedi trust the will of the Force keeping the balance, while Sith abuse and distort it. Sith cannot experience Oneness because they do not surrender to the Force.
At no point in the book Malgus experienced oneness. Towards the end of the book he became more powerful by killing his girlfriend, which amplified his anger levels and made him an ultimate Sith but it has nothing to do with oneness with the Force.
Just a little comparison:
Malgus:
"NIGHT, and controlled rage, wrapped Malgus. His anger smoldered always now, and his thoughts mirrored the caliginous air.
...
Thinking of Eleena blew oxygen on the embers of his anger. In life, Eleena had been his weakness, a tool to be exploited by rivals. In death, she had become his strength, her memory the lens of his rage. He resided in the calm eye of a storm of hate. Power churned around him, within him. He did not feel as if he were drawing on the Force, using it. He felt as if he were the Force, as if he had merged with it. He had evolved. Nothing split his loyalties any longer. He served the Force and only the Force, and his understanding of it increased daily.
The growing power whirling around him, leaking through the lid of his control, made the suppression of his Force signature impossible. All at once he lowered all of the mental barriers, let the full force of his power roil around him.
...
Malgus felt the hate pouring off Adraas, the power, but it paled in comparison to the rage and hate roiling in Malgus. In his mind’s eye, he saw Eleena’s face as she died. It poured fuel on the flames of his rage."
Anakin Solo:
"Anakin opened himself to the Force completely, drawing it into himself through the power of his emotions - not through his anger or fear like a Dark Jedi, but through his love for his family and his fellow Jedi Knights, through his faith in the Jedi purpose and the promise of the future. The Force poured in from all sides, filling him with a swirling maelstrom of power and purpose, saturating him and devouring him. There was nothing to be frightened of, no reason to grieve. He could feel it flowing into him and himself flowing into it. Anakin was the Force, and the Force was Anakin.
Anakin rose. His body emitted a faint aura of light - the glow of his cells burning out - and the air crackled around him. His injuries no longer pained him. He was acutely aware of everything in the grashal - the musty smell of the droning thud bugs, the sultry heat rising from the planting bins, the huffing breath of his fellow Jedi, even the Yuuzhan Vong. Their presence was as distinct to him as that of his own companions, almost as though the Force had somehow expanded to include them.
...
It no longer hurt to speak, but his aura had gone from faint to bright. His cells were burning like fire. "Jacen, you're in charge. Take everyone and go after her."
...
A plasma ball roared through the grashal door and set fire to a twenty-meter swath of cloning vines. Anakin charged back toward the melted membrane, miniature forks of lightning dancing off his arms and legs, the Force swirling through him like fire, burning more ferociously every moment. He was completely filled with the strength of the light side now; his injured body could hold no more. The energy was burning its way out of him, consuming a vessel too weakened to contain it."
Indeed, as Nephtys pointed out Malgus seems to be closest among the Sith to experience Oneness. He even felt it like surrendering to the Force. Yet, it still pales in comparison to any Jedi that experienced Oneness. Anakin as example was literally burning out.