Harry x Hermione

The day after Harry is entered into The Tri Wizard Tournament, only Hermione supports him, the duo looking through the library for survival methods/spells, etc.

Hermione finds a spell/ritual, Caelum Porcis(Hogs Heaven) which triples the magical power of a male and double for female, while making the female incredibly spell resistant and increases both of their confidence, with all that is sacrificed is the users virginity afterwards and the appearance of the female. The duo resolve to do it, Harry promising Hermione that know matter her appearance, he'll always care for her.

Hermione has Harry cast the spell on her, as the spell/ritual instructs. Before their very eyes, Hermione fattens up incredibly quickly, becoming blob like in a few seconds, forcing her into a sitting position as her shirt rides up her breasts, and her skirt is swallowed by her bum and tummy, which extends far past her feet and spreads across the floor.

Before the ritual takes control to finish, Harry tells Hermione she's beautiful and asks if she will marry him, which she says yes before the duo lose themselves to the magical hormones.

Next morning, and the following months, Harry and Hermione have to deal Voldemort, Dumbledore, Ron, Molly, the racism of Magical Britain, the problems being impossibly obese bring(even though they both enjoy them) and the small, bisexual harem of witches that is forming for the two of them.