The Cursed Crystals is an event that can occur in the Infested Ruins in Diablo III.
Upon entering, the Nephalem will find several Cursed Crystals that can be activated one by one. Destroying a crystal will cause a horde of Shadow Vermin to spawn, along with Warscarred Marauders or Warscarred Ravagers. Destroying the final crystal spawns Keeper Hashemel himself.
Upon killing the Keeper, a Cursed Chest becomes available, and the event is complete.
This event is one of the two featuring these devices: The Crystal Prison in Pandemonium Fortress level 2 features identical devices.