Found in The Catacombs, in a coffin not too far past the first fog door, hidden in a large room behind a breakable wall. The path out of the room also leads to a ladder leading to the Tranquil Walk of Peace miracle and a hidden shortcut to the room where the second necromancer is found.
The Darkmoon Seance Ring grants one additional attunement slot. The Darkmoon Seance Ring is identical to the White Seance Ring in this regard; the two can be worn at the same time for two additional attunement slots in total. Wearing this ring in front of the Gwyn statue located in the Darkmoon Tomb bonfire room in Anor Londo will reveal a passage leading to a fog door. Approach it and Dark Sun Gwyndolin will offer you entrance into the Blade of the Darkmoon covenant.