Thought Key

Thought Magecraft’s version of a Magic Crest.
They are access rights to a vessel—the Thought Foundation.
They are passed down from teacher to student, and unlike Magic Crests, aren’t restricted to families and bloodlines. The magecraft inscribed in this Key is like marks painted in a bowl. The bowl remains unchanged but is observed differently depending on what’s painted on it. Long story short, the whole world can be deceived by this. Just watch out for when the world retaliates.

However, inheriting a Key means constantly taking influence from the “bowl”. One who inherits a top-class Key and succeeds at fully drawing out its abilities is called a Tianxian and is past halfway through a process of fusing with the world. Those who managed to reach this point gain a level of influence somewhat similar to that of a Ghost Liner, but at the same time, struggle to maintain the self-identity necessary to brandish their powers.
Side note: It’s possible to access the Thought Foundation without a Key, but the usable magecraft is limited to the basic spells (It’s still possible to achieve advanced spells by compiling basic spells together). The access rights provided by the Keys are to a form of privileged domain. This is similar to how the spells contained in a Magic Crest are, almost without exception, secret arts directly connected to one’s family.

TYPE-MOON BOOKS material: The Adventures of Lord El-Melloi II Glossary