Your old bullies..

It was easy locating them as they would always be sulking around in the park where Dudley would hang out. You brought in Mrs Figg with you transformed as Aunt Marge. The boys immediately fell on her feet. You pick them up and at Mrs Figgs place treated them to the "special milk" before explaining the plan to them. The effect was instantaneous, they now preferred You over Marge thus the initial part of the plan was a success.

Mrs Figg then transformed into her milfy form which made them too excited. They calmed down after they had an orgy session which Mrs Figg enjoyed thoroughly.

After explaining the plan to them for what must be more than 10 times (If they get it right, they have fun with Mrs Figg else their cocks were chastised). This made Mrs Figg bloat up due to the amount of semen inside her. She vomited them out and pushed out as much as she could with your help and she retired in your arms in the form of a teenage girl.

The boys then carried out the plan where they lure Marge to another house of a Muggle family who was happy to oblige unless you impregnated the muggle's wife. You had your fun with her(Mrs Figg was slightly disappointed though she played with the husband) and they promised to name their son after you.

You then lie in wait back at Mrs Figg's house until Marge was locked in.