The Holy Tomb is unlocked by visiting the Amiibo Gazebo during any NG+ run. If you go to the Holy Tomb during the academy phase, Sothis will be there waiting for you. Talking to her for the first time will grant Byleth the ability to use Sothis Shield. Sothis Shield can be used once per battle, and grants your entire army a buff to resistance for three turns, starting at +7 and decreasing each turn.

Using Sothis Shield will take one weapon durability off of whatever weapon or spell that Byleth had equipped when using it. This (potential bug) is not documented anywhere in the game.