How do you get silverlight on RuneScape

To get the Silverlight you will need 3 keys, below I have placed the guide that will help you get them after you have done the part that leads to the keys ( Guide from Runescape Wiki ):

First Key

In the north-west corner of the castle is a set of stairs. Climb them twice to reach a room with Captain Rovin. Speak to him and tell him that a demon is going to invade the city, that you have to defeat it, and you need his Silverlight key. Select the option that says "Sir Prysin said you would give me the key" and then select the option that says "Then why did he give you the key?" After bringing up Sir Prysin's clumsiness he will hand it to you.It will take some arguing, though.
Second Key

The key is stuck down a drain behind the kitchen in the castle. Go get a bucket or jug and fill it with water at any water source; the sink in the kitchen is close and convenient (if you did not bring a bucket, up the stairs there is a room right behind you with a bucket and theres a bucket in front of the drain just outside the kitchen). Use the bucket of water on the drain and the key will be washed into the sewers. Enter the Varrock sewers using the manhole located immediately east of the castle's keep. Follow the north-western path until you get to a stream and search around there. It is just beside the skeletons under a pile of mud. Search for a rusty key on the ground; it will not appear on the minimap. Grab it.
Third Key

Gather 25 sets of regular, unnoted bones. You can buy them on the Grand Exchange or kill almost any monster and take its bones. Head to the Wizards' Tower, located south of Draynor Village. On the first floor (up one stairs) of the tower, talk to Wizard Traiborn about the key, and he will ask for 25 bones. Give them to him and he will perform a ritual, then he will give you the third key.