User blog comment:B214/General discussion/@comment-119.94.177.7-20160416124127/@comment-139.195.58.103-20160417043137

In Vol 10, it is explained:

Tsukikage speaks with Ikki and Stella at some point. His first point is about the process of becoming a Majin: essentially, the strong desire to surpass one's inborn limits is a critical factor in allowing one to become one. After you do become one, even magical power becomes like a physical muscle, and will improve by training. But it is not something you can intentionally seek - it must be au naturel, or boom you're dead. Seriously. Which is why the existence of this process is a closely-guarded secret: no one wants people popping left and right in the pursuit of power. Nangou and Saikyou are both Majin.