Legendary Chest In The Mountain. A legendary chest can be found in the river pass during the second mission path to the mountain shortly after meeting the witch. To find these winds drop down to the level below and break apart some hel bramble on a room to the right to find a receptacle you can carry.
After fighting with the dragon on the mountain sindri will give atreus shock arrows. I noticed that i collected 2 of 3 legendary chests in the mountain. You must detonate the bomb to reach it.
Once you find the mine cart needed to make a lift up the mountain in the central lift room push.
During the mission path to the mountain. When you can get it. Legendary chests of the mountain. Legendary chests in foothills.