its very simple but it took me at least 2 hours and 2 playthroughs to get it.

you need to find the sacrificial tree (somewhere north-east of where you'll spawn, youll find it within 5 seconds ure in the scene), you'll notice there's a stone inside wood fence surrounding the tree.

with that stone, head to the direction of where the stone is pointing to. you'll notice this weird rock formation surrounded by trees and bones. get in and youll find the chest of the tomb.