"Squeak—Squeak—"
Under Richard's watchful gaze, the gates of Black Castle slowly opened, followed by an unbearably grating noise that sent shivers down one's spine, forcing one to imagine various scenes: nails scratching glass, chalk screeching on a blackboard, keys scraping the bottom of a rusty iron bucket...
Richard's eyebrows deeply furrowed, fully aware of the cause of this phenomenon.
Clearly, the sound of the door opening contained a special frequency that could stimulate an inherent response in the human brain.
This inherent response in the human brain originated from ancient instincts. In the era of Upright Apes without language, humans used to emit this specific frequency of sound as a warning to alert their companions of approaching danger and to escape quickly.
