The Interdict of Merlin is a magical bond over the entier world that forbids powerful spells to be transmitted except from one living mind to another, and in particular to learn such spells from written text.
It is thought that the Interdict more specifically targets spells that could threaten civilization or lead to wide-scale catastrophes.
There is no known way to directly disobey the Interdict of Merlin. However, Salazar Slytherin (canon) circumvented it by teaching his lore to a basilisk (canon) hidden in Hogwarts's legendary Chamber of Secrets (canon). Thanks to the snake's very long life, it could teach what it knew to Slytherin's parselmouth (canon) descendants who managed to find the Chamber of Secrets, such as Tom Riddle.
- Binding magical contract on HP Wiki
- ↑ Does the Interdict of Merlin appear in original Harry Potter canon? (Word of God on StackEchange)