Entering Emily's brain.

I immediately go for it. I close my round mouth and press my mollusk face into the little hole, stretching my slimy flesh to the limit to fit into it.
So far, the process of passing from the nasal cavity to the bigger one where the brain is at is the most excruciatingly slow of everything I've done so far. I have to move my body in a way that causes minimum damage to the nerves, blood vessels and Emily's grey matter. It feels incredibly crammed, but I still patiently make my way towards the front and then to the upper part, always following the deep crevice between the two hemispheres of the brain.

And once I'm finally 'in place', once I've arrived to the point I somehow know I have to be, I turn my freaky head down, open my slimy mouth, and allow my three longer tentacles to quietly sink straight into the grey matter.

From then on, I relax completely, comfortably nestled into the most tightly secured place of the whole human body while allowing my natural instincts to do the work. I don't know what is happening, but I can somehow feel it. I feel my thin tentacles connecting to the nerves at different points of the brain, blending around the girl's braincels and growing their own smaller tentacles, which also branch into even smaller tentacles.
All those microscopic appendages start spreading out slowly but steadily, like very little tree roots, using the braincell connections as improvised pathways to know where to grow and seamlessly blending around the blood vessels, from which they feed their growth.

Of course, Emily's body doesn't stay still against this catastrophic intrusion. Even though I'm tightly placed between the skull and the two brain hemispheres, I can still feel the movement of the girl twitching violently. But there's nothing she can do at this point. If there is a little intruder poking at your brain from the inside, it's not like you can just reach into it and take it out. At this point, not even the greatest neurosurgeon in the planet could take me out without killing my host.

So I keep this on, calmly extending my existing tendrils and sprouting new ones until my original three tentacles have branched into thousands.

But this is just the beginning.

Emily's immune system initially tries to fight off my intrusion, however there is something in the chemicals released by my tendrils that completely shuts it off. Later on, it even starts working against her, helping me speed up the assimilation.

I myself have no idea how this is happening, other than it's an unfathomably complex and precise process. It's not like I'm controlling anything directly. This process is just something my new body is fully prepared for. Just as cats can make it so they always land on their legs and owls can fly without making a single sound, this mutant slug body of mine has been genetically conditioned to be able to take over a human brain... a task I perform flawlessly, without giving my new host a single chance to fight back and without harming her more than necessary.

At some point, the violent twitching fully stops, and Emily's body seems to calm down.