The Brecilian Forest. There will be 3 tombstones you'll need to disturb. Then you'll fight one Revenant per tombstone along with a few skeletons.
You only have the option to disturb the tombstone nearest the Hermit first, then go back to the others in the forest to disturb them. There are two gravestones before the mist, one past the mist and the final piece inside the ruins.
When you're inside the ruins in the Brecilian Forest you will be led to a quest called The Elven Ritual which will open a door leading to the shade of an ancient elven woman. She'll attack you along with a few other shades. Defeat them, look in the coffin for the final Juggernaut piece.