Here's a guide on how to do it. From the Hinterland Site of Grace, follow the main path across several Tree Sentinels and a bridge until you reach the Hinterland Bridge Site of Grace. Keep going ...

Both are on dead bodies that you must loot. The Tree Sentinels at the Hinterland also drop Blessings of Marika every time they die; there are two of them near the Site of Grace, so you can kill ...