The seventh chest is not a chest at all. The seventh chest is a metaphor. when you defeat atheon, you get the chance to celebrate with your team and realize that you could not have killed him without them. And that my friend, is the greatest reward of all. So in short form, our victory is the seventh chest...
English
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As plainly as that sounds I thought something similar. Atheon stands in the middle so a while I thought he was the chest but the last line of the riddle is what changed my mind.