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.

Chief Elder Kanai will rise during the month of March and invite you to join him on a quick run through his Stomping Grounds. During the final week of March (March 24-31), completion of this event has a guaranteed chance to spawn a chest containing the Kanai’s Skorn transmog. Why the final week of March? Kevin Kanai Griffith, the artist to whom this event is dedicated to, celebrated his birthday in the last week of the month.

This item is specifically used for Transmogrification.