[jolem]

For the horror aspect: I think the most terrifying thing that would happen to a victim feeling their own head transform into something inhuman, especially if it's the last thing to change. At the moment they finally understand that they are no longer human or the same person they were anymore.

The other horror aspect of transformation I can think of if there are mental changes happening to the victim as well. The original victim's mind could be fighting the new version every second of it as a new mind emerges from the transformation. It's almost like they're drowning while the new mind surfaces. It gets even worse for the victim if they're forced to watch the new mind takes control of their body.