Arthur had a destination in mind when he left the restaurant. He heard about a certain Kang Treasury house which is a well-known shop that sells treasures, artifacts, and weapons. Many humans visit in once they come to Dark Desert City as it is rumored that it has many secret techniques and high ranked weapons.
Secret Techniques have different kinds, some use lifespan to grant the user a boost of power or speed, and others are life-saving measures which can protect their users from certain death. Such techniques are extremely rare and once they show up, a lot of people covet it and fight for it.
In a way, secret techniques are similar to Physique Manuals, however, there are only one-hundred and eight physique manuals altogether, as for secret techniques, there are probably way more than that.