This quest is unlocked by completing the quest “In the Queen’s Image” (https://youtu.be/Joh1lIZ7aH0).
Update: (!) now castrum gives FIVE loathsome memories!
Shadowbringers Relic Quest Playlist: https://www.youtube.com/playlist?list=PLAIwJ_87EVatVH51HDvwwZOQdUcJxcTNV
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Just getting back into the game but how do you unlock this quest? I got my recollection weapon and that's it