FOUND! It's Secrets by potter8668. Thank you so much everyone!

It's been deleted but can be downloaded from Fichub. One of the commenters has kindly provided instructions.

I'm looking for a specific Harry Potter fanfic. The story takes place at Hogwarts. Harry secretly brings a baby to Hogwarts. He finds some rooms (maybe founders rooms, not sure about that) and is able to keep the baby secret but Hermione finds out. I remember the baby's accidental magic is a little less 'accidental' and he/she keeps escaping his/her room and making his/her way to Harry's bed. At one point, Ginny uses an enhanced lust potion at Dumbledore's prompting. It's supposed to be impossible to resist but Harry manages to resist long enough to hole himself up in his rooms with Hermione, who has sex with him until the potion wears off. Harry insists on marrying Hermione first (or immediately after, I'm unclear on that point). I remember, at the end of the story the child is abducted to Voldemort, but when Hermione and Harry arrive to rescue him/her, he/she is using accidental magic to keep Voldie in his chair/throne and changing him into different outfits. Voldie is begging to be rescued.

Please help me identify this fic as I really want to read it again!