The Earth Fiend Evil Spirits are adept at the Earth Fiend Technique!

However, the path of the earth veins is incredibly profound, and earth vein resonance is the most advanced aspect of the Earth Fiend Technique.

To put it bluntly, Guili, the Earth Fiend Holy Son, only heard about it when the Earth Fiend Evil Lord and the Great Elder discussed it.

If you ask a regular Earth Fiend Evil Spirit about earth vein resonance, they would probably tell you they have no idea what it means!