Hall of Dark Arts appears sometimes in the Westmarch Commons in Act V. Once inside the house (the place is called "The Survivor's Enclave") you will see a Resplendent Chest and Methan the Necromancer who you can talk to. The Necromancer's Choice event can be triggered after the conversation with the Necromancer.
Access to Levels
Events in Hall of Dark Arts
Common Monsters found in Hall of Dark Arts
Notable NPCs
Dialogs in Hall of Dark Arts
Nephalem: "Why are you here, necromancer?"
Methan the Necromancer: "I've come to uphold the balance. Death's minions threaten to overwhelm the living, so they must be culled."
Methan the Necromancer: "This beacon will draw many of Death's soldiers here to their demise. I was about to activate it."
Nephalem: "Allow me."
Methan the Necromancer: "It has been good fighting by your side, but I have much work left to do. Perhaps we shall meet again someday."