Silverlight key is one of the three keys needed to open the three locks of the sealed box which contains the sword Silverlight, a crucial item in the quest Demon Slayer. It is acquired from Captain Rovin after the player has started the quest and has asked Sir Prysin for the location of the keys.
By talking to Captain Rovin and choosing correct phrases to say, he will give the 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.