From Diablo Wiki
Kanai's Skorn was an event added in patch 2.4.1 in 2016 and take place every March to celebrate the life of Blizzard artist Kenneth Kanai Griffith who sadly pased away. He was also the inspiration for Kanai’s Cube.
The item drops from a mysterious chest in the Immortal Throne. The Immortal Throne can be found in the Ruins of Sescheron in Act III through Adventure Mode and is situated north of Kanai's Throneroom located on level two of the Ruins of Sescheron in Act Three.
This item is specifically used for Transmogrification.