Found in its unknown form (Strange Amulet) in Hjerim, hidden under a pile of Beware the Butcher! pamphlets on a small shelf during the quest 'Blood on the Ice'. Sell it to Calixto during the quest and it can be looted off the corpse of the murderer in its final form (Necromancer's Amulet) at the end of the quest.
The display can take both the original (only the final form) and the replica.
You need to have the original (either the unknown or the final form) in your inventory for the replica recipe to be available.