This scene seed is a mutt coming from Heather Hansen's prompt on Flashy Fiction, Nathaniel Lee's "Inspiration", Radiolab's episode on parasites, and the character of John Fortune from the Wildcards shared universe.
Tony burrowed under the host's flesh while it slept. Nightmares would haunt the host but it wouldn't wake as it twisted and turned as Tony crept just under the epidermal layer leaving an itchy trail laced with drugs to keep the host asleep. It took time. Finally, Tony hunkered beneath the host's forehead and lowered the drill to cut through the skull. Through the hole, he deployed the knotted nerve endings, anticlotting agents dripping from the woven tendrils. The nerves were part of him and he unwove the ends into dozens of strands that sought the important nerve centers of his host so that he could integrate himself. Each of the parts of the brain, occipital lobe, parietal lobe, hypothalamus, cerebellum, pituitary gland, had a different taste and Tony sought out each of the desired endpoints. It took time, Tony must be certain before locking the hair-like ends of his nerves into place with the host.
Tony connected with the host's ears and aural circuits first. Tony felt the host's dizziness as it stood. More than just the wooziness imparted from the drugs he had secreted. The host moved too fast, fearful. He heard running water and felt the host scratching at its forehead trying to rip Tony out. Tony hunkered down, plastering himself against the skull and concentrated on getting the other nerve endings attached.
"... look at that thing. It's as big around as my thumb. That hurts, I can't get rid of it, I can't pop it like a pimple, I wonder. I should call the doctor. No, I must get rid of it. A knife. Yes, a knife, I'll cut it out..."
Tony had linked with the hosts memories. The host was frightened, they always are, it was difficult to follow the host's thoughts as they chattered in the background. Tony felt the host sway.
Patched into the host's eyes, Tony cringed as he watched the host paw through a drawer and grab a knife, the eight inch blade catching the bluish white light from the hallway. It stumbled back toward the bathroom.
"Cut it out, but it will hurt. Don't matter. Must, cut it out must get rid of the thing, must do something now, must..."
"Stop, slow down." said Tony. He used his nerve to introduce his own thoughts.
"What, I must be going crazy, who's that..." The host's thoughts careened through its head and Tony struggled to follow a coherent thread. He needed to complete the parasitic connection so that he had full control over the host's motor functions. He watched the host stumble into the bathroom. He needed to teach the host tolerance, before the host spitted Tony upon its blade. Didn't the host realize that it would kill itself with Tony's death?
The host's head reflected in the bathroom's mirror and it tilted its head to lean the forehead forward where it could see the red welted bump with bloody scratches etched in the penumbra underneath the hosts black hair. The knifetip set against the skin. The pressure of the knife pushed on Tony's carapace.
The host paused and the knife lifted off the skin for a moment. "Why? Who are you? What are you?"
Toby's last nerve strand settled into place and he lowered the host's arms. "My name is Toby, and I'll be with you for a while."