The so-called forbidden techniques can be understood as advanced versions of magic, similar to divine techniques, known for their great power.
The grades of forbidden techniques, like Taoist techniques, range from low to high as follows: beginner, intermediate, advanced, exquisite, mysterious, sect-stabilizing, supreme, and grand.
In situations where grand is almost impossible to see, supreme is the highest version, but given his current situation, it's definitely not possible to acquire a supreme forbidden technique. Sect-stabilizing level would require him to become a true inheritor of the sect before he might have the chance to learn it.
So, the mysterious level is basically the highest level of forbidden techniques non-true inheritors can obtain.
"This needs to be upgraded again, it's a powerful method."
