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. It follows the life of Sayo Yasuda and the days leading up to the Rokkenjima massacre. Confession is also the only story wherein Sayo directly appears without any form of disguise. [Wikipedia]
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.