I Bring Dire News
As a High Priestess, Cai had developed a skill that would allow Spirits to Possess her for short periods of time when she was still very young.
This allowed her to harness their powers and use it to defend herself or to attack others in self-defense.
Through the years, she had mastered this ability, allowing her to retain the spirits inside her for a long time that could last for several days up to several weeks.
When Queen Rhiannon possessed her in the hope to corrupt and control her body from within, the former didn't know this.
She only succeeded to possess Cai in the beginning because the former allowed her to succeed.
Cai was certain that Lux would come to her rescue, so she confidently laid out the foundation of her trap that would prevent the Succubus Queen from escaping once she possessed her body.