Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Bee and Puppycat
Wallace's transformation/possession, where the monster woman pours out of his mouth. His eyes turn wide and red, sharp teeth appear in his mouth, and then the fact that her appearance is accompanied by gross, wet sounds.
The monster herself, with two large, fanged mouths that cover her face, her vein covered grotesque body, and five sharp spines coming off her back.