This is story-related and cannot 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 Seath the Scaleless.
To reach Seath you'll have to traverse through the Duke's Archives and then through the Crystal Cave. From the main Anor Londo bonfire turn left and after passing a couple of guards you'll reach a slope with a fog door leading to the Archives. This is one of the fog doors that will only open after acquiring the Lordvessel.
Inside you'll quickly reach a room with Seath the Scaleless himself, however at this point you'll be unable to beat him and will be forced to die, so plan accordingly. After dying you'll appear in a bonfire strained from others in the Archive's dungeon. Push your way through the rest of the area until you go outside and enter the Crystal Cave, a treacherous place with many paths that are invisible and the only way to locate them is to watch the falling crystal shards. Whenever you see one hitting something mid-air it means there is a path there.
At the end of this area you'll reach Seath again and finally this time around you'll be able to kill him, get his soul and get the trophy.