mages treasure

I'm not sure which one holds the Mages Treasure Chest Plate so I'll give you the rundown on all three locations. All of the pieces of armor are found somewhere within the Brecilian Forest.

The first is the tombstone in the West Brecilian Forest which is near where you first fought with some darkspawn and an orge on the way in. Very close to the exit.

The second piece is in the East Brecilian Forest location, just past the misty barrier and among some of the ruins.

The final piece is actually within the Brecilian Ruins. You'll find the armor piece within a sarcophagus inside the room you solve the Elven Ritual quest. Only after you solve that Ritual will you gain access to that room with the sarcophagus and then you'll have to fight a shade to reach it.

If you don't know how to fulfill the Elven Ritual, I'll give you the steps here as well.

Find the Elven tablet that's located in one of the side rooms very near to where you encountered the ghostly boy near the start of the level. Then you need to go to the Elven Ritual room and perform the steps in the correct order. Remember, if you get a step wrong, you'll be attacked by shades.

First examine the fountain to start the dialog.
Fill the earthen jug with water.
Leave the Pool alone.
Examine the altar to start the dialog.
Place the filled earthen jug on top of the altar.
Kneel before the altar and pray.
Examine the earthen jug on the altar.
Take a single sip from the water in the jug.
Take the earthen jug.
Leave the altar alone.
Examine the fountain to start the dialog.
Dump the water in the jug back into the pool.

If you do the steps in order correctly, the large doors in the room will open, revealing an elven burial chamber. Inside, a shade guarding the sarcophagus waits for you to get closer. Take the armor from both this quest and the Mage's Treaure to get two of thebetter armor sets in the game.