Deathly Vision

Rui had figured out the secret by process of elimination. Just the speed of death meant that poison and bioweapons were highly unlikely.

It had to be something that directly triggered death and absolute death.

He could rule out most forms of death.

Even something like a heart attack death would not be so smooth and instantaneous.

There was, however, one option that Rui knew that could kill people instantaneously.

The Voidreaper's Death's Symphony.

Of course, the attack had grown obsolete in the Master Realm simply because Masters could hamper vibrations in their bodies rather easily. Their mastery of medium principles of science was passively a lot higher simply by virtue of being in the Master Realm.

However, this comparison led Rui to identify what was most likely the broader means by which his opponent triggered death with a simple touch.

The brain.