You can't become a demigod. It's impossible, you also sound like a Percy Jackson fan. Also there's no spell for that!
The myths of the demigpds were myths, and made up. People believed in them, for it was there tradition. Did it actually happen? No one knows, are they real? No one really believes that.
What you want is a fantasy spell. Those don't work, and honestly to be a son of hades, he's a god of the underworld, gems, jewels, death, ect. You would have to be born from him!