From Diablo Wiki
The objective of Necromancer's Choice event is to help Mehtan the Necromancer defeat three waves of reapers in the Hall of Dark Arts in the Westmarch Commons in Act V. The Reapers (Summoned Archers and Summoned Soldiers) appear once the Necromancer's Beacon is activated. The content of a Resplendent Chest is your reward. Necromancer's Choice is one of the events that must be completed to obtain the Sightseeing and The Tourists Achievements.
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