The Hall of Souls is where you can open Soul Stones and access the Secret. You can access the Hall of Souls by either tapping the blue portal of the Soul Stones icon.
Opening Soul Stones Edit
Either double tap or drag the soul stones to one of the 9 opening slots and press the 'open' button. Double tapping will fill the first available open slot so you must drag a soul stone if you want to complete the daily secret code.
If you open 9 soul stones at the same time there is a possibility that a slot that forms part of the daily code will be highlighted.
Daily secret Edit
See the page on the Ancient secret for more information.