This is story-related and can't be missed.
Once you acquire the Lordvessel you'll be given the task of filling it with the four souls of the great lords of the world so that you can finally challenge the game's final boss. One of these souls is the soul of the Four Kings.
The Four Kings are located the very bottom of the New Londo Ruins, in the Abyss, however to get there you'll need to first lower the water of the area and also have equipped the ring Covenant of Artorias (check the "Art of Abysswalking" trophy description for information on how to get it). The Abyss is located by jumping inside a deep hole in the area and the ring prevents you from dying instantly with the fall.
To lower the water of the whole area and clear the way to the Abyss entrance you'll have to release a seal that opens the huge doors that connect New Londo to the Valley of Drakes, however the seal is inside a locked door and the only NPC of the area, Ingward, is the one with the key.
To get to him you'll have to go through the narrow bridges and ghost-filled buildings in order to reach the roof of the biggest one where you'll find him. If you talk to him after acquiring the Lordvessel he will promptly give you the key, however if you get to him before doing so (one of the ways to enter the Darkwraith covenant) you'll have to kill him in order to loot the key, release the seal, and progress to the Abyss entrance.