I don't believe that the Nightmares could be considered the Champions of the Darkness. I don't think the Darkness ever got around to replacing Oryx. The Darkness is pulling a Calus on us. It wishes for us to become it's champions.
English
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At this point savathun or quiria have taken his spot Xivu wants to become independent of the worms and therefore the darkness so it’s not her
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Wrong way around, Savathun is the one who wants independence. Do also keep in mind the Taken throne is not the same as the Darkness's champion, remember that Oryx sought to embody the Final Shape in all aspects
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Ya realized I was wrong after I went and read more into it