This guide will aid you in achieving 100% completion rate for Arlathan Forest.
Q&A before starting
A: There are a total of 11 normal chests, 2 Lyrium chests and 7 Altars.
Q: Can I start as soon as I reach Arlathan Forest?
A: Yes, however, I recommend to start after you’ve completed the following quests as they unlock paths to the majority of the level which will make it easier to collect in one go.
1. Echoes of the Past (Bellara)
2. How to train a Griffon (Davrin)
3. An Unfamiliar Sense (Harding)
Be aware that you can collect only 10 normal chests, 2 Lyrium chest and all altars with the above aknowledges. The rest will unlock once you reach Bellara’s and Davrin’s second outing, so keep that in mind.
Keep in mind that the chests were made in no particular order, I just took screenshots as I explored Arlathan Forest.
Chest #1
This will unlock after you’ve completed Harding’s first Outing.
Puzzle:
To unlock the door, go behind the structure and use Bellara or Neve (or the dagger if they’re not in your party) ability to break the seal. 
Chest #2
From previous chest, move north to the map, it’s the next chest appearing on the map below.
Puzzle:
You will need to move some stones with Harding’s ability and reach atop.
Chest #3
Puzzle:
First, take this crystal from its socket.
Move forward after the barrier goes offline and place the crystal in this socket to unlock path to the crystal beam.
Use the crystal beam thingy and move it to the far right to have a path to the other crystal beam.
After that, take the crystal away again and place it to the socket behind it.
After that, push down this balance beam to unlock another path to the crystal beam.
Remove the crystal from the socket again.
Reach the crystal beam thingy from the other side of the balance beam and point the crystal beam to this socket.
Now remove the crystal from its socket to remove the barrier.
Chest #4
From previous chest location, move north again to this location.
Puzzle:
Use Bellara’s ability to repair the socket.
Then reach this beam on the other side and point it at the repaired socket. A path will be created to reach the chest.
Chest #5
This chest is found at the beach in the middle northern side of the level.
Puzzle:
Use Bellara’s ability to repair this socket and place the crystal in the repaired socket.
Doing this will place a barrier that will block the crystal beam.
Chest #6
Once you reach this place in Ruins Reach, go right of the Wolf statue and there you will find the chest.
Chest #7
Reach this location on the Ancient Aqueduct.
And move across to this location. It’s a pretty straight forward path.
Chest #8
This location will open after Davrin’s first outing, so you can reach this chest right after completing that.
Chest #9
This chest is located near chest 7, so go back to the Ancient Aqueduct again.
Puzzle:
Go to this zipline.
Cross it and take the crystal off of its socket to take the barrier down.
Move the crystal to this crystal beam and activate it (it’s left from the previous location).Now move the beam to this socket.
A new path will be unlocked to reach the chest.
Chest #10
Chest #11
Lyrium Chest #1
Reach this location in the Waterfront Glades.
Puzzle:
Take this crystal away.
Place the crystal in this socket to activate the crystal beam. Point the crystal beam to this socket.
After that, move to this second crystal beam and point it to this socket to activate another crystal beam.
Now move where this new crystal beam is and point it towards this crystal socket.
Now move to the third location found here.Activate the crystal beam and point it towards this socket.
The final product should look similar to this and a path will unveal for you to reach the Lyrium Chest.
Lyrium Chest #2
Reach this location in the Tainted Woods.
Puzzle:
In this building, you will need to destroy all the blight pustules.
After that, you will need to reach this area.
Once you destroy the blight around the frame, a path will open up to reach a chest.
Altars
And that wraps up our share on Dragon Age™: The Veilguard: Arlathan Forest – 100% Completion Guide. If you have any additional insights or tips to contribute, don’t hesitate to drop a comment below. For a more in-depth read, you can refer to the original article here by D34Done, who deserves all the credit. Happy gaming!