@SunRazer
Post-Nathema Vitiate is referred to as Lord Vitiate only in the context of Ritual of Nathema and immediate developments after that. For example:
Lord 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. Lord Vitiate declares himself Emperor of the Sith. Under his sovereign rule, the Empire retreats into deep space to rebuild its strength and prepare for vengeance.
Taken from Star Wars: The Old Republic: Encyclopedia
However, same source makes this point afterwards:
The ritual lasted ten days. Lord Vitiate orchestrated the sorcery and the planet Mediraas was consumed by the largest dark side nexus the galaxy would ever see. 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 Mediraas became a void in the Force and was erased from history. From that moment forward, the world would forever be known as Nathema, birthplace of the one and only Sith Emperor.
Taken from Star Wars: The Old Republic: Encyclopedia
The aforementioned statement corroborates the view of Darth Nyriss:
"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."
Taken from Star Was: The Old Republic: Revan
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More importantly, we have revelations that suggest that Post-Nathema Vitiate is a being unique in the galaxy; an entity who embraced different identities for different civilizations such as Sith Emperor for Sith Empire and Immortal Emperor for Zakuul Empire. At this stage, he is no mere Sith Lord.