Talk to Sir Ebal Assasin. He will tell you that an old buddy of his has news of a deadly force threatening Lumbridge and to deliver Elvarg's tooth to him. His buddy's name is Velgar and he is probably in the Jolly Boar Inn. He takes the tooth right off you and then ignores you. A stranger named Awtn12 comes along and tells you to get his attention you will need a Black night deathdifier and he will sell you one for 1000gp. You buy the drink and hand it to Velgar he then tells you to deliver the Notes of Ivel to Sir Ebal Assasin.