The event Cryptology (associated with the Campaign achievement, Tomb Wader) appears in Act V of Diablo III in Briarthorn Cemetery. A woman by the name of Lady Gwyndolin stands outside of a crypt and claims that a monstrosity has infested her family's crypt and there is no one to protect it. A swarm of Shadows of Death will appear and must be fought off, after which the Chilled Reaper, Eremiel, steps in as the boss for the event. After his death, Lady Gwyndolin will reveal herself as a ghost and she and her family finally rest in piece.
This event was unable to be completed for quite some time due to a bug that made it impossible to interact with the Lady. However, as of January 2017 and patch 2.4.3, the bug has been fixed and the event is now accessible.
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Diablo 3 Reaper of Souls Cryptology Event