In Game Description
Key to the floodgates of New Londo, which seal away the Four Kings who fell to Dark.
The Sealers flooded New Londo to banish the Darkwraiths and the Four Kings. The agonizing decision was made with the realization that countless lives, and the robust culture of the city, would be lost. The victims now roam the ruins as ghosts.
There are two ways to obtain this key
The gate that gives access to the lever controlling the giant door holding back the water in the New Londo Ruins