Buck Testa

Spoilers for Star vs and a theory

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So I think Glossaryck is not dead per say. What I think is that he can only exist in the "Age of magic" or whenever his magic exists. Time travel and time manipulation are very common things in the shows canon, and its shown he can freely traverse the flow of time as long as his magic is there. And it is HIS magic. It is his realm of magic, the temples are made in his image, and he said he'd go down with "his ship" talking about the realm of magic. 

The reason he is so laid back yet so calculating with every single thing he ever has done in the show is because he's done it all before. He's run the path of the Age of magic many times, and with each run he knows how to train up each princess and set the stage for when the age of magic ends. He is in effect running a centuries long version of Groundhog Day over and over. He's lathered hints about him knowing how everything will turn out throughout the series, so it really would not be a stretch to say that he's just been running this rat race for longer than anyone can fathom.