It is not advised to read this without completing Umineko no Naku Koro Ni Chiru.
Confession of the Golden Witch is a manga-only arc contained within the Twilight of the Golden Witch manga. This fan-adaptation of the manga chapters in a VN medium contains multiple stories: Confession of the Golden Witch, Abuse of Power, The Cage of Obligations, When the Seagulls Cry.
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