The number of side tasks in "Assassin's Creed: Shadow" is very large, and the side tasks of the mind and body is one of them. This task is one of the Nine Characters Mantra series tasks. The completion location of the task is on the cushion on the waterside stone near the Hizuki Shejin's hiding place. Just operate it in the past.
How to do Assassin's Creed Shadow Heart and Body Side Quest