Danger of Custom Core

"Their competition lasts for a few years until they reach twelve." said Jayna.

"Twelve?" asked Angus confusedly.

"Yeah. Apparently, when Aunt Extalia reaches twelve, she gets into an accident trying to form her core. By a sheer coincidence, she manages to form a core that is not only better than ordinary but boosts her physical level each time she breakthroughs." explained Jayna.

"She what…" exclaimed Angus in surprise.

As someone who has researched and tried to create a custom core, Angus knows the difficulty and danger of forming a core. Just being a better core than those at the royal academy is already difficult. 

Now, Jayna said the Duchess manages to create a core that will boost her physical strength each time she breakthroughs at twelve. This kind of core is not impossible to form in theory. But, to actualize, it is almost impossible with almost zero percent of success.