Now I think the reason oryx thought they died true deaths was because of his limited power and understanding.
Now with what we as guardians did, in the book oryx clearly listed out everything that had to be done to make his death permanent.
Now unless he has a spare echo roaming around somewhere, oryx is truly gone.
English
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He knew about these throne worlds very well because he pulled his inside out to move his into the Dreadnaught.
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This was after their deaths though and after he gained a greater understanding. Also he only killed one of their forms. He didn't hunt down any possible echoes or other forms they might have had.
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Okay, I may have missed that, or forgot where that took place. I may have to read it again soon.