Memory alteration is one of the practical skills I learned in the past timeline. It was a skill that I learned secretly when I went to a black market in the game and one vendor teaches me to do this ability in exchange for some information he needed which I currently owned at that time. This ability is usable in real life and can be used as long as you possess mana in your body. There are no other prerequisites just to learn it other than having mana in the body. Because it was a skill that can be used for the wrong purpose, only a limited number of people know about it to avoid letting evil-minded people know this.