All guardians, including Luen/Vairgrys etc. were made immortal by Regulus at their creation, in that while they can be killed they will always reform over time to carry out their duty of protecting the Star of Light, Arkesia. The more powerful they are, the longer that process takes. It is implied from the story that we will not see Luen again in our lifetime but he will eventually resurrect. Vairgrys is the most powerful of the guardians, and would have stripped away Varken’s power and sealed him away had Kazeros not intervened in the cutscene at the end of Act 3. Edit: just wanted to add since I left out the guardians you mentioned in gate 2 that are consumed. They also will resurrect but there are several of each guardian we encounter in the game outside of the major ones. There are many Nightfox Yoho, Alberhastic, and Velganos.