excuse me. . . Ahm. . . I am also using RPG Maker XP i tried examining my game to see if I can help with your problem. . . and I think it is better for you to create a state called Demonize. . . then create a skill that will trigger that state. . . because in the states column of the database there is a selection there for the release condition of the state or in your case,
the skill. . .
I hope you try this and tell me what happens. . . :)