I highly doubt this is due to her bargain with an Ahamkara.
As for the culprit, this Scarlet fortress appears to have been in the moon all along, perhaps something was inside which Eris disturbed? There's a reason the new raid is in the Black Garden...
English
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<The Scarlet stuff is new. The Tower was NOT there before. But there IS something that could've been there since the Golden Age...>
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We have seen Ahamkara's doing things like this before, like Riven. Yes, the fortress was there during the golden-age, but it has been re-awokend. Perhaps with a wish? About the raid: They said in the ViDoc that there were a lot of locations they could have chosen for the raid, so I believe that the raid has nothing to do with the story (or nothing big atleast) and is more like a CurseOfOsiris or Warmind kinda raid, which had Leviathan raids.
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There's a big difference between Riven, whose wish was made from 6 god-slaying reality-bending guardians and Eris's, who wasn't even a guardian anymore at that point.