Necromancer Amulet (Skyrim)

Initially called the Strange Amulet, it is found on a bookshelf in Hjerim in Windhelm as part of the quest Blood on the Ice.

Obtaining the Amulet

 * 1) Sell the Strange Amulet to Calixto.
 * 2) Go to Wuunferth, the court mage and he'll identify it as the necromancer's amulet. He will suggest patrolling the streets at night for the Butcher.
 * 3) Wait till night and start walking around the slums and market place. You'll see Calixto attempting to murder a woman, do not stop him.
 * 4) Wait till it says "catch the killer" and Calixto will head back to the Hjerim.
 * 5) Kill Calixto in Hjerim and search search his body for the Amulet

Bugs

 * 1) Accusing the Mage of being the murder will make you unable to obatin the Amulet
 * 2) If you do not sell the amulet to Calixto and kill him as part of Blood on the Ice, it will be stuck in your inventory as a quest item. [confirmed]
 * 3) If you accuse Wuunferth of being the butcher, after a unknown amount of time you can go and talk to him in his cell. He will then tell you where the butcher will commit his next murder.