AN: This follows Chapter Fifteen of HBP; some parts are direct quotes, some are paraphrases and the rest is of my own invention - what I think might have been going on in Snape's head in that chapter.
Chapter One: Bah Humbug!
Severus Snape had never hated Christmas as much as he did this year. It was bad enough that he had to work with his old professor, but he now found himself standing irritably in front of his tiny smudged mirror, straightening his collar.
I'm only going because someone's got to keep an eye on that blasted vampire, Severus told himself.
Years ago, he would have been delighted to receive an invitation to Slughorn's Christmas Party, if only so he could blatantly reject it. No, he thought bitterly, he would have gone in the end – for her.
Oh, what's the point? Severus wondered angrily and spun away from his reflection.
He stalked toward the festivities, nearly rolling his eyes at the music and laughter spilling from the room before him. He entered and slunk along the wall toward a darkened corner where he planned to spend the majority of the evening. He maneuvered through the room to the hazy corner where the smoke from a few elderly warlocks' pipes hung in the air. He was so intent on his destination that he tripped over a house-elf carrying a silver platter and its edge banged sharply into his shins. He looked down in pained fury. The house-elf emitted a high-pitched squeak and bobbed away.
Severus settled into his corner, folded his arms and did his best to blend in with the shadows and smoke around him. Slughorn was in the middle of the room in a ridiculous tasseled velvet hat, greeting each guest loudly. Severus noticed know-it-all Granger arrive with that great hulk of a Gryffindor who rivaled Longbottom for Potions disasters. They said their hellos to Slughorn and McLaggen promptly drug her off in the direction of the mistletoe. No doubt she was trying to make Potter jealous by showing up with that idiot, but Severus suspected Potter would never give the bushy-haired bookworm a thought beyond next week's Potions essay.
"Harry, m'boy!" boomed Slughorn, alerting the entire room to the presence of Potter, who had the Lovegood girl on his arm. Severus wondered briefly whether Potter had lost a bet, or if it was some sort of joke. In any case, the girl seemed oblivious. Lovegood was another Potions disaster waiting to happen – she spent so much time daydreaming in class that she often came dangerously close to exploding her cauldron. However, she had an advantage in being a Ravenclaw – she was smart enough to correct her mistakes before anything serious happened. Of course, that didn't stop Severus from noticing and deducting points for carelessness. The poor girl had obviously fallen under Potter's spell, and had even followed him to the Department of Mysteries to duel with Death Eaters. Severus found himself glaring at the back of Potter's head. Thoughtless, arrogant, boy.
There was Eldred Worple and the vampire. Severus watched them intensely as Slughorn introduced them to Potter. The vampire was eyeing a group of girls nearby while Worple prattled on. Severus tensed and moved to intervene, but Worple grabbed the vampire and shoved a pasty into his hand. Idiot. If he had ever really lived among the vampires, Severus was a candied yam.
Potter finally broke away from Worple and disappeared into the crowd. Severus stared out at the party in thought. So this was what he had missed all those years. Of course, he would have hated it, but it had been absolutely insulting to be left out, especially when he had been the best Potions student at Hogwarts then (and since then, he was sure). He was beginning to feel quite resentful again, when his thoughts were interrupted by Sybill Trelawney, or rather, by the strong smell of cooking sherry. The woman was mumbling to herself and clutching a goblet in her hands. Severus decided some things were worse than mingling, and standing in a smoky corner with Sybill Trelawney was one of them. He slowly moved away, thinking it might be a good opportunity to keep a closer eye on Sanguini the vampire. He didn't get very far, because Slughorn saw him and immediately began introducing him to any and every guest within arms reach, including Eldred Worple.
"Severus! This is Eldred Worple, I was telling you about - and old student of mine, author of-"
"Shouldn't you be entertaining your friend?" Severus interrupted, giving the stout little man a pointed look.
"Oho! Now Severus, I told you not to worry about that!" Slughorn motioned toward the vampire, "Sanguini is a guest and a friend!"
Worple paled, excused himself and pushed his way through the crowd toward the vampire and the five girls who had gathered around him. Slughorn only laughed and scanned the crowd again.
"Come on Severus, I see Harry Potter and Miss Granger have taken pity on poor Sybill! We can't let her have them all to herself!"
I'd take the old bat over Potter any day, thought Severus, and watched Slughorn sidle up to Trelawney. He planned to grab a goblet and find another secluded corner, as there was no one fit for conversation (even if he had been in the mood to talk). He saw a house-elf walk by with drinks and tried to follow it through the crowd. He thought he had escaped Slughorn's notice as he slunk by, but he found himself hooked by the arm and unceremoniously presented to Potter's horrified face. Granger had vanished, and Lovegood looked as complacent as ever.
"Stop skulking and come and join us, Severus!" hiccupped Slughorn happily. "I was just talking about Harry's exceptional potion-making! Some credit must go to you, of course, you taught him for five years!"
Severus wondered just how much – refreshment – it had taken to make Slughorn believe Potter could make an exceptional potion. Perhaps it was just flattery to ensure Potter's attendance at the next party.
"Funny, I never had the impression that I managed to teach Potter anything at all," said Severus, enjoying the look on Potter's face.
"Well, then, it's natural ability!" shouted Slughorn. "You should have seen what he gave me, first lesson, Draught of Living Death – never had a student produce finer on a first attempt, I don't think even you, Severus-"
Severus silently vowed to a find reason to give Potter detention on the night of every Slug party until the end of the year. This outrageous flattery was unbelievable, even from Slughorn! Potter would no sooner be able to brew the Draught of Living Death than Sybill Trelawney! "I don't think even you, Severus" rang in his ears.
"Really?" was all he said, quietly, staring Potter down. He noticed the boy looked very uncomfortable.
"Remind me what other subjects you're taking, Harry?" asked Slughorn, apparently realizing that Severus would not go along with his game. Potter started listing his classes.
"All the subjects required, in short, for an Auror," said Severus, holding back a sneer. When Minerva had congratulated him on finally getting the DADA position, she had also smugly informed him that she was glad Harry Potter would no longer be denied his life's ambition because of a less-than-outstanding Potions OWL. Surely Slughorn's star pupil could have met my standards, thought Severus.
"Yeah, well, that's what I'd like to do," said Potter defiantly.
"And a great one you'll make too," boomed Slughorn.
"I don't think you should be an Auror, Harry," said Lovegood. Severus stared at her along with everyone else. At least someone in this group had some sense. Of course, the next words out of her mouth were some nonsense about a conspiracy to bring down the ministry through Dark Magic and gum disease. Severus kept his face carefully blank as Potter snorted into his mead and Slughorn looked bewildered. He noticed the girl was unfazed when Potter laughed at her. Suddenly, Potter looked past Severus and his expression changed to one of smug delight.
Severus turned his head and saw Argus Filch approaching with Draco Malfoy - in fact, dragging the boy by the ear. What had the spawn of Lucius Malfoy done now? Severus hoped it was something that might warrant a detention rather than something that would compel him to act on his vow. In any case, he might finally have the opportunity to drag the foolish boy away and question him.