Kirnardaz let his dark soul expand and gain somewhat of a foothold in hers, allowing her control, and keeping his form. He smiled, and his mind boomed out in hers.
"I am not threatening you... You are interesting to me... I am a bored Demon Prince... Here, let's have a listen to my soul to prove it..." He reached into her soul and powered it up, forcing her to latch onto his soul to survive. It was usually something demons did for fun, soul swapping. On mortals it could be forced, unless the mortal had a strong will. Hopefully she wouldn't.
He let the voices and screams flow from his soul and into her mind. The combined noise was everyone Kirnardaz had crushed the souls of. The loudest scream was his mother's. The loudest shouting was his father's. She could hear them all saying things.
"No! Please! Noooo!!!"
"Nonsense, why would a demon possess you?! You're pathetic! Now put the knife down..."
"Not my baby, please! Pleeaase!!! NOOO!!!"
"How could you Kirnardaz? I raised you! I bore you into this realm! You ungrateful little demon! You would try and crush your ow- AAAAGGGGHHHH!!!"
"You have come to kill me like you killed your mother Kirnardaz? Tell me... You think you ca- Oh gods... No... Kirnardaz, NO!!!! PLEA- AAARRRGGHHH!!!"
Kirnardaz shut the flow immediately after that, feeling she had seen enough now. He was crazy perhaps, and no doubt she would try and run. The chase would be fun though.
"Now do you believe me? That's more than enough proof for me being a demon, hahahahaha..."