Eris will contact you through the Crown of Sorrow once she has formulated a new plan, and she encourages you to continue to explore the Derelict Leviathan and contain its Nightmares in the meantime.
This quest will progress at the next weekly reset.
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This is an unexpected victory. Eris has successfully performed a ritual of severance on Calus's connection to the lunar Pyramid, but that hold has not broken entirely. Eris needs time to study this new force you created from the Nightmare of Uldren Sov before she can advise on a further course of action. In the interim, it's best to help by containing the Nightmares aboard the Leviathan.