Silverlight key is one of the three keys needed to open the box where the magical sword Silverlight is sealed in.
This key belongs to Sir Prysin, the other two given to Captain Rovin and Wizard Traiborn. He has dropped it into the kitchen drain of the castle. To get it back, the player must get a bucket of water, by getting an empty bucket and filling it up with water, and using the bucket of water into the kitchen drain (not the kitchen sink). If you lack a bucket, the nearest empty bucket can be found in the castle. Just go up the stairs in the kitchen, and an empty bucket is sitting next to a bed in a room by the stairs. You can use the kitchen sink to fill the bucket with water.
This will cause the key to fall into the sewer and the player must go to the Varrock Sewers, Go to the west passage near, the river is the key. When the mouse is moved over the key, "Take rusty key" appears in the corner of the screen.