This manga somehow fails at world building in this part. The guy is telling him his skill is one of 3 skills his master, a Sword Saint who's already retired (perhaps no longer playing the MMO), supposedly solely knows. The sword saint taught the guy one of the skills, while MC used another of the three. He explains that MC's skill was once used by another disciple and became addicted to it, as it grows stronger the more it takes lives. Then the sword saint killed that disciple and retired.

Where does this fail in world building you ask? This is literally an MMORPG world they're talking about. MC was able to choose the freaking skill at character creation and it was known as a weak one at best. There's also nothing stopping the disciple from returning to his last saved point after death. And no real reason why it would only be these dojo folks who would know of MC's skill when literally anybody can pick it up during character creation. The presentaion of the past events treats it too much as a real life event so the pieces don't hold together.