How Can One Cultivate Immortality Without Seizing One's Body?

I died and I lived again. There is an old monster in my mind who has been imprisoned for 30 million years. He said that I was a soul outside of heaven, not controlled by heaven, and could take over others at will. I didn't take it seriously. Until the only person I don't need to play dies. Now I want to become a saint. The old devil said he would help me. What he didn't say was that he had been waiting for this day for 30 million years.