Perin gets possessed by Neir.

As you clean the jizz out of her cunt with your tongue, Perin begins to cry.
"Please stop... I'm begging you..." The bound woman pleaded you.
You swallow the hot cum. "I think she's ready for you, Neir..."
"As you wish, ma'am..."

Neir steps up close to Perin. You reach up, take his head in your hands and kiss him deeply.
"Remember, Speak your spell-word clearly, focus your mind and keep your self centered." You say to him. He simply nods.
As you step back to give him room to work, Neir begins. His eyes flash and the lanterns on the walls grow dim. A cloud of blue smoke begins to form in the room, swirling about both Neir and his prey. The foxes panic grows as she yells at the both of you.
"PLEASE! LET ME GO!"
The smoke coalesces into a ball in Neirs' hand. He shouts "REND!" and shoves his hand between her breasts, passing through her skin. Perin lets out an ear-piercing screech like none you've ever heard.
"AAAAAAGHHHHH!!!"
Her body bucks and jerks as bolts of vibrant electricity shoot about the room. Soon, the girls protests cease as Neir wrenches his hand from her chest. As he does so, her body falls limp, appearing to be dead in every sense.
Neir holds up the girls' soul and studies it. The ball of smoke is now a ball of pure blue fire. It dances in his hand, dousing his face in azure light. He gestures toward you.
"Mistress, could you bring that lantern over here?"
"Of course."
You bring it over to him. He opens it up, removes the candle and replaces it with the soul. He closes the lamp tight and has you put it back on the wall.
Then, he prepares for the next step of the process.

He grabs hold of the empty bodies neck, holding her mouth open for him to slip inside. His eyes flash and he says "OCCUPY!" He is then surrounded by green smoke, swirling around his head. Then, his body begins to transform into smoke itself. Starting with his horns, Neirs' smoky body begins to **** itself down the girls throat. He fills up the girls body from the inside.
As the last of the stuff slides into her mouth, the foxes head drops. Her body, once limp, now jerks and jitters as its new owner asserts its dominance.
Soon, the head shoots up.
"OH, MY GODS!" It says in its thick accent.
She takes a series of deep breaths.
"Milady? Are ya still there?"
You step up to the captive woman, barely containing your excitement.
"I'm here, Neir. Are you okay?! How do you feel?!"
She licks the inside of her mouth, feeling her new teeth.
"I feel... fookin' fantastic!"