Silverlight key is one of the three keys needed to unlock Silverlight from its sealed casing during the Demon Slayer quest. At the start of the quest, this key is in the possession of a wizard named Traiborn, who is residing at the second floor of the wizard's tower. Traiborn will ask the player for 25 bones in order for him to summon his secret wardrobe (Examine reads, "A wardrobe brought forth by magic"), which contains this key. Once he summons his wardrobe, Traiborn will hand over this key to the player.
Traiborn's secret wardrobe.