Let me introduce the use of demon souls in 2 areas.

1/ Merge
2/ Forge

1/ Merge

You can merge the demon souls to get some demon souls that are more difficult to capture in the wild.

Merging souls requires soulstones.

If you have all the components, tap OK to merge the souls.

If you don’t have all the components of the new soul, tap gain and pay a premium price for the soul.

2/ Forge

You can forge the demon souls into your weapons too, and it requires soulstones to do so.

If you have all the components, tap forge to combine the souls with your weapons.

If you don’t have all the components of the new weapon, tap gain and pay a premium price to get the weapon.