Void Essence was a simple Essence that on its own did nothing. It was an amalgamation of Essences that remained in a harmonious state where it could turn to any other Essence it needed to.
When it came into contact with anything that had any sort of Essence in it, it would turn itself to the exact opposite of that Essence, consuming it, making it vanish.
It was called a Void because everything that touched it disappeared, becoming a form of Essence that was simply pure.
Ning's Void skill created a domain around him that destroyed any and all Essence, including the skills that were used. As such, when he used it, Saburn could not use anything to protect himself.
Ning landed on the ground and next to him was the now-dead Saburn.
No one expected Saburn to die. Why would they? He was fighting against someone that was 2 realms below him, so there was no threat to him.