Like an organ for magic. They don't have a dantian so they instead focus all the energy into a single physical core. Strengthening themselves by storing more energy, and enhancing their ability to use it not through complex techniques but instead aborbing and storing as much energy as possible.
Edit: After rereading a few times... I don't uh. Think I understood. Like... sex the magic monster core?
...sex the monster or just the mana core?