VMMM…!
In an extravagantly large isolated training chamber in Daracol, Rui meditated in absolute solitude, radiating intense Yin vibrations.
It had taken him more than a year just to reach a stage where he could generate a meaningful amount of Yin vibrations. It was a slow and tedious process. He had to gradually condition his mind to enter a state such that it would cause metabolic vibrations in cells indirectly through the endocrinological system.
Adrenaline and endorphins surged through his tissue, exciting his cells and causing them to accelerate metabolically, producing more vibrations.
He trained this very phenomenon with his domains and Sympathetic Death Lance, teaching his cells to vibrate hard enough to negate the incoming vibration with destructive resonance.
Most of them couldn't do that.
Thus, most of them died.