Spiritual skills are the skills that can directly affect your soul and mind. Earlier the rhino used a spiritual skill that was directly able to affect his mind.
According to what Evan knows the power of spiritual skills depends on the strength of your soul.
Evan himself doesn't know much about soul and spiritual skills since there is not much information available about them.
But he does know the stronger your soul is, the more powerful your spiritual skill will be, and if you use spiritual skills against someone whose soul power is stronger than you, then there is a very high chance the skill won't work on him and you might even receive a backlash because of it.
Before meeting Rhino Evan never encounter someone who used a spiritual skill against him.
But after facing the rhino Evan started to doubt that his soul is quite weak.
When the rhino used its skill he felt like his head was about to burst open.
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