After defeating Hraezlyr during the journey mission “Inside the Mountain” and getting the shock arrows,, you’ll come across a sap-covered room. The shatter crystal pod is located in a nearby altar, worshipped by reavers. Destroy the reavers and take the shatter crystal. Throw it at the sap and detonate it with Atreus’ shock arrows. This will allow you to access the shrine inside the room.