Disclaimer -- I own nothing. JKRowling owns it all.
It was early evening, the days' O.W.L.s had finished and the wizarding examiners were sitting in a conference room in Hogwarts discussing this years talent. Professor Tofty was bragging to the room at large about the patronus that Harry had cast.
Catching her attention, Madame Marchbanks asked him to repeat what he'd said.
"Yes, well Harry Potter cast every defensive spell perfectly, then for extra credit he cast a corporeal patronus."
Madame Marchbanks thought this was interesting since she'd been informed by Dolores Umbridge herself that this year's defense students were severely lacking in skill and discipline. "Anyone else have anything extra to report? ... No. Okay, I'll see you all tomorrow morning."
Making her way to the visitor's quarters she'd been assigned, she spotted the Golden Trio in the hallway ahead of her. "Mr. Potter, a word if I may."
Sending Ron and Hermione back to the common room, Harry turned down the hall to Madame Marchbanks. "Good evening ma'am. May I help you?"
"Yes, I was just told the most amazing story by Professor Tofty about your defense practical. He said you were able to cast all the spells perfectly and had cast a corporeal patronus. I heard about your trial last summer and would like to see your infamous patronus if I could."
"Ummm ok. Filch gets mad if we do magic in the halls so how about we use the charms classroom on the next floor."
"Lead the way, Mr Potter."
As they entered the classroom, Madame Marchbanks sealed the door behind her "Don't want to be interrupted. Professor Umbridge told me that your class lacked skill and discipline so I'd just like to see for myself."
"Yes, Professor Umbridge and I don't exactly get along. It's true that we didn't do any spellcasting in class, but some of us managed to find some free time and an empty room to practice in to be able to pass our exams. What would you like to see first?"
As Madame Marchbanks had him demonstrate defense spells from 1st year thru O.W.L. level and even a few that were N.E.W.T. level she cast a mild compulsion charm she'd created herself, a mix of trust-me and lust, on the classroom walls. The charm was being slowly strengthened by each spell that Harry cast until she finally asked him to cast his patronus.
"Expecto Patronum" and Prongs burst forth from Harry's wand, slowly making his way around the room, looking at each wall, the floor, ceiling, and the desks. Finding no dementors, Prongs returned to Harry for a quick pat on the head before disappearing.
Harry turned to Madame Marchbanks, and instead of seeing a 180 year old gray haired wrinkled witch, his eyes clouded over and he saw a vision of nubile beauty. More ethereal than any Veela, more hypnotizing than any vampire, more seductive than a dozen Cho Changs he was hers and didn't know it.
"Strip" she told him. "I want you. Your power is intoxicating, invigorating." As Harry began to strip, she transfigured two desks into a king-sized bed before stripping herself.
Pulling Harry over to the bed she asked him "Have you ever done this before?"
"No. I haven't. I don't know what to do."
Laying down next to him, she proceeded to teach him the ways of men and women. Waking him periodically throughout the night to continue her instruction.
As the cold gray light of dawn peeked in through the windows, Harry slowly awoke with a feeling of weight and warmth on his right side. Looking over, he saw the wrinkled brow of Britian's oldest living witch. "What? Why?"
"For over 100 years, I've been on the wizarding exam authority. During that time, I've sought the number one, the most powerful wizard in each O.W.L. year. This year was your year, Mr Potter. You are a very powerful wizard. You're the new number one."
"But why?"
"Did you enjoy yourself?"
"I don't remember everything but what I do remember, yes."
"Sometimes we just have to do things because we enjoy them."
"What do I tell my friends?"
"Whatever you want but you won't be able to tell them about this, it's part of the spell so you'll need to make something up."
"Do you remember everyone?"
"Oh yes. I remember them. Tom Riddle used to be number one. Tom was a strange boy. Hissed a little bit like he was in pain or something. And I think he had a girlfriend. Molly, Millie, Myrtle, yes Myrtle. He kept moaning her name."
"Albus might have been the old number one. I taught him transfiguration you know. The rules are different for teachers than they are for the rest of us. I wouldn't have been able to try out any of the students I was teaching without getting in trouble."
"But why not wait until after N.E.W.T.s?"
"Magical maturity begins on a wizards 16th birthday with a gradual increase in power until the 17th when it comletes. Waiting until after N.E.W.T.s gets different results based on whether or not a wizard has completed maturing. Right after O.W.L.s is best. The magic is pure and shows the true potential of a wizard."
"Who, ummm, who was it in my father's year?"
"That would have been Sirius Black. He and your father were of similar power levels but Sirius was a bit better in defense. He was another strange one. He was already experienced and had a certain fondness for doggy style. Time to get going Mr Potter, you've a long day ahead of you."
fin