There's an achievement for completing a raid without anyone in your Fireteam dying at all... that means not one person can die, no one can get spotted by a Gorgon and you cannot succumb to the Orcales.
If we can complete this, I think the final chest will spawn in Atheon's Arena.
What do you all think?
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It was suggested to me by a friend that, provided no one dies and the vex do not take control of a sync plate, there would be a second chest that spawns once you raise the Spire & open the Vault. He showed me a path directly underneath the very first chest that takes you through a jumping puzzle to reach the Templar area. We didnt find anything whilst down there but there it is all the same.