>>11408504
Girl tied up or chained down with a symbol drawn on her belly. Immediate pregnancy, huge, before she can begin to think or struggle, tentacles/worms/etc spill from her pussy, ass, mouth, and nipples. Over time, her eyes change color to blackened sclera. Nipples get dark and grotesque, and her legs mutate into either a lamia tail or a mass of tentacles (she still has an ass and pussy to birth from) Waves of growth and birth until she blacks out.

(Painting a picture of the scenario. However you wanted to draw the story is up to you)