User blog comment:B214/Question & Answers 5/@comment-82.199.209.134-20160426111856/@comment-27876048-20160428114014

Idk it could be a situation similar to Rakudai Kishi no Eiyuutan and they ability to use magic or demonic power comes from their souls (that's at least my interpretation). They could draw energy from their life-force and its converted to magic or demonic energy.