How to Get the Kanai’s Cube 

Kanai’s Cube (the KC or Cube) was added Patch 2.3, It harkens back to the Horadric Cube of Diablo 2, giving players the ability to convert or break down legendary items and gain bonuses from their special properties.
The cube is obtained from the Kanai's Throneroom located on level two of the Ruins of Sescheron in Act Three. This area was added to the game in Patch 2.3, along with the Cube, and is found only in Adventure Mode. Players need not complete a quest or bounty to obtain the Cube; just find the area (which requires some exploration, as it's not marked on the map and the Ruins are large)