The Trial of the Sword is included with the Breath of the Wild Expansion Pass.

How to Unlock the Trial of the Sword

To unlock the Trial of the Sword, you will need to purchase the Breath of the Wild Expansion Pass via the "Downloadable Content" option on the Title Screen, complete The Hero's Sword quest, and leave/re-enter the Lost Woods. A voice will tell you to speak with the Deku Tree. When you approach the pedestal where you pulled out the Master Sword, a cutscene will occur. Afterward, you can begin the Trial of the Sword any time by putting the Master Sword back into the pedestal.

Trial of the Sword Guide

Please see the Trial of the Sword guide for the walkthrough.