The Magic was originally the "Forgetfulness Technique (Tier Three)."
It was a Magic that could only be used on oneself, allowing the user to forget things more quickly.
'What bored Mage invented a Magic like this?'
In fact, this world was full of useless Magic, which was why so few spells were in wide circulation.
If you looked closely, you'd find that many spells were both difficult to learn and utterly useless. It was hard to see the point in learning them at all.
But in truth, they weren't entirely without purpose.
For instance, right now...
When reading these Forbidden Books, this Magic was essential.
Even if he read some forbidden knowledge he wasn't supposed to, he could just erase the memory and pass out. After waking up, he wouldn't remember a thing.
It was an effective way to avoid the dangers of forbidden knowledge.
