In order to summon Lord Valthalak we're first going to need to gather the other two-thirds of the amulet. To do that we need to get a brazier and some special coals to burn in it. There are three nobles of fire, Lord Incedius in Blackrock Depths, Pyroguard Emberseer in Blackrock Spire and Executor Asharell in the Steppes, with what we need. Collect from each a piece of their remains, and then return to me with a Hallowed Brazier, which can be purchased from the Argent Dawn if you are honored by them.