Dark Moon Ring is a Key Item in ELDEN RING (2022).
This item is part of Ranni the Witch's questline.
Ring depicting a leaden full moon. Symbolic of a cold oath, the ring is supposed to be given by Lunar Princess Ranni to her consort.
Ranni is an Empyrean, meaning her consort would by rights earn the title of lord.
A warning is engraved within; "Whoever thou mayest be, take not the ring from this place, the solitude beyond the night is better mine alone."
Area: inside the chest near Rennala, Queen of the Full Moon that requires the Discarded Palace Key to open.
Firstly, it dispells the seal found after Astel Naturalborn of the Void that leads to the Moonlight Altar of Liurnia, then it allows the player to marry Ranni below the Cathedral of Manus Celes, unlocking her summon sign after the final boss which triggers the Age of the Stars ending.