it looks to me that sandra is the demon/evil thing in the prophecy and she uses her dark magic to manipulate people into believing her and having clouded judgement, her dark aura may be harmful to the actual saintess Elaine, considering she got disproportionately hurt by Evan who was poisoned by Sandra's black magic, it might argument bad traits, like strengthening Evan and making him more violent, making the prince more obsessed with Sandra, making little bro more weak and stealing the healing abilities of others whilst making them sick. That's my current theory.