Amused by him, the demon chortled and clapped Wanda's flopping hand against her bloody one in sick applause. "Give the man a prize! He figured it out." Horror, manipulation, wouldn't break this one, it realized. Tony had his facts and logic as defense, the will to force his world into something that made sense. Well that was just fine; the demon liked a good game. That was all the possession, this ordeal, really was to it. The beast inside Wanda was simply there killing time.
Lurching around the room again, Wanda's body paced until it bent to pick up something from behind a table, her back to Tony. "I do need this vessel, and as long as I'm in it, the body can't die." When it turned back around it was holding the silver knife that Jessica had thrown across the room in their earlier fight. It drew the blade in a thin line across Wanda's neck and collar bones, smeared blood on its hand to lick off.
"And now you know. But so what? If you attack me, you just hurt her. Try to get me out, and I'll hurt her." To prove this point, it raised the knife and stabbed it down into Wanda's thigh, bringing her to the fore long enough to let out a strangled cry and faint. The demon caught her body from the inside in mid-collapse, rising slowly from the freakish angle of her fall back into a teetering upright position, the blade still embedded in her leg.
It continued as though there had been no interruption, "If somehow you do manage to get me out of here, she will die.." The monster twisted Wanda's features into an exaggerated look of contemplation. "So how do you fix this, Toooony? What's the solution to your problem?"