Magic Crest

An inheritance passed down within magi families.
The mysteries that a magus has spent his entire life establishing (stabilizing) are turned into a crest and left to his descendants.
It’s no exaggeration to say that the crest is engraved with the history of the entire bloodline, and that the magus who inherits it is burdened not only with his family’s regrets, but is duty bound to pass it on to a successor as well.
In a way, you could call it a curse passed down through the generations.

Fate/side material: Fate Encyclopedia

Passed down through the generations along with a family’s history, the greatest treasure of any magus bloodline.
Like an additional organ added to the body, it won’t be compatible with anyone from outside the magus’ family. In the world of magecraft, the reason older families have such an overwhelming advantage can be narrowed down to primarily because of the number of magic circuits, and the existence of these Magic Crests. The magecraft engraved within that Crest serves both as a magus’ greatest power, and as a curse binding them until they pass it down to their own descendants.
At present, El-Melloi II’s Magic Crest is being held by Melvin. Though some people may have witnessed it for the first time in the last episode of the anime, it is truly nothing more than a “square frame.”
In short, there’s nothing inside of it at all. In exchange for there being no curse tied to it, it contains nothing but what future generations might fill it with. That’s why El-Melloi II is so hell bent on recovering that Magic Crest.
“It’s the number one piece of collateral that guarantees a magus cannot betray you. Taking that away from them is like taking way all the meaning their lives ever had, after all.”

Lord El-Melloi II Case Files material: Lord El-Melloi II Case Files Glossary