Where do you find Juggernaut Armour in Dragon Age Origins

The Juggernaut cuirass is in the Elven Ruins, in the Brecilian Forest. You have to complete a small puzzle involving a pool, a chalice and an altar, to open the area containing it.

The helmet, gloves and greaves are in the possession of three Revenants, two in East Brecilian Forest, one in the West. Their resting places are marked gravestones, you can interact with them to disturb the ruins and start an encounter with a Revenant and some assorted undead. Once the Revenant falls, the Juggernaut piece is there for the taking.