McGonagall wasn't going to let the "hoss" thing go, and had even tried to get me to volunteer to be turned into a horse to demonstrate human transfiguration for our class Monday morning. I'd demurred, but had honestly considered it because of the good mood I was in: we'd just gotten there off of a defense class that hadn't been awful. Lockhart had us doing some in-class dueling, building toward more silent and focusless casting. He hadn't really presented any keen insights, but he wasn't actively being an impediment and that was exciting.

After lunch was our once-weekly potions class, scheduled to take up the whole afternoon. It had lost more students than any other core class I was still attending, including most of the more annoying Slytherin quidditch team members. Unfortunately, Oliver had also dropped it, so I was down a lab partner. While Percy clearly wanted to take the opportunity to partner with Penny, he'd gotten into a good rhythm with his fellow prefect, Alexis, Oliver's girlfriend. So that left me partnered with Penny.

Professor Belby's bespoke potioneer's robes were as immaculate as ever, but his hair had gone full mad scientist over the summer. Still sticking up all over from his nervous habit of running his potions-covered hands through it, a substantial chunk on the left side was gouged out with a hint of black still on the tips, as if he managed to catch his own hair on fire recently. As usual, he didn't notice.

"Welcome to NEWT potions," he lectured, once we were all settled. "Most of you did very well on your OWLs. Here it all changes. You have copies of Advanced Potion Making. It's the classic. We'll barely be using it, so there was no point in getting a new text. Well, we'll be using it as a stepping-off point. This year is about improvisation. We'll learn what you can change about recipes to get a different result. What you can substitute and still get the same. Next year, we'll be inventing potions from first principles.

"I have up here the standard components of the Draught of the Living Death," he gestured at several containers on his desk. "Find the recipe in your books." I did and found that the recipe was as thoroughly annotated as everything else in the text. "Now, let's talk about why it's written the way it is, and how the classic recipe can be improved…"

During the following discussion, I wound up scoring several points by explaining the sopophorous bean's reaction to pressure and silver as a way to extract juice, and why periodic alteration in stirring direction better melded the ingredients. While both made sense from my previous knowledge of the subject, I was admittedly prompted by the handwritten notes in my textbook. Penny had noticed early on, read over my shoulder, and snagged her own points for Ravenclaw by pointing out that a more arithmantically-significant number of beans, 13 instead of 12, were relevant to the concoction.

With the longer class, Belby wound up giving us a break after the discussion, and Percy immediately pulled his chair over to our station. "Excellent contributions, both," he said, then noticed we were busily flipping through my textbook. "That is a… heavily annotated book."

Penny nodded, grinning, "Looks like Harry got the secondhand from someone very talented at potions. Lots of good modifications and notes in here."

I caught a page before Penny flipped past it and pointed, "Not just potions. This is a whole arithmancy matrix done in the margins. Looks like he already took it to a spell creation. 'Sectumsempra' is what, 'Forever cutting' or 'Eternally severed?'"

"Could be both," Percy worked out his own Latin translation. He tried to read the small handwriting upside down, "What does it say next to it? 'For Enemies?'"

I nodded, then added, "Looks like a modified severing charm." I'd been getting a lot of use out of that charm with all my leatherworking of the last year.

"Oh, my," Penny breathed, taking in the arithmancy for the spell. "I've seen a lot of these constructs in our work with the fiendfyre interface. It doesn't have the hellfire core, but I think this basically takes the severing charm and turns it into a pretty dark curse."

Percy had moved all the way around, leaning over Penny's shoulder so he could get a better look. "I think so too. And notice the missing and altered factors. This should have increased range and… Harry, do you remember what this part does in the severing charm?"

I looked where he was pointing and let out a hiss of breath, "In the severing charm that's the piece that ensures it's far more effective against dead organic material than living, so you have to try really hard to cut your own finger off. This flips it. This will cut off your finger but be much less useful for craft work. And I bet it deals curse wounds."

"What kind of student dreams up something this vicious and just says 'For Enemies?'" wondered Penny.

I shrugged and said, "Called himself the Half-Blood Prince." I showed them the "property of" note in the back of the book.

"Pretentious," Penny scoffed.

"Well, this book seems old enough that it cannot be active evidence in any crimes," Percy thought out loud. "But we may still want to inquire with the DMLE whether this curse has been used." Showing how much less of a narc he'd become over the last year, he then added, "Quietly. And perhaps after we have fully examined the rest of the text for context."

"And any useful arithmancy work that's not gross or illegal," I nodded. While I was the last person to run to the aurors with a book, I didn't really want something as nasty as this spell getting out without people being aware of it, and I didn't want to be left holding something that could be pinned on me if it had been used to kill people.

Penny had been flipping ahead and said, "Well, at least he was responsible enough to invent a counterspell: 'Vulnera Sanentur.' Maybe it was just a thought exercise? Looks like this might be an even better general spell for cuts than the basic healing charm."

At that, Professor Belby came back out of his office, and the break was over, so Percy headed back to his desk and Penny let me take control of my textbook back.

Since potions was the last (and only) class of the afternoon and ended just before dinner, we'd planned to check the book over further in the library after the meal. We'd met back up outside the great hall and were on the way when I heard an unusual noise and asked, "Do they actually have radiators in the castle?" They both looked confused so I explained, "Sounds like hissing pipes, up through the wall, like when someone cuts the heat on."

"Well there are pipes in the walls," Percy nodded. "I believe most of them were installed as part of a class project in the 1940s when they decided to modernize the toilets, which was no small feat. But they are only for plumbing, not for heating."

"Yeah, I didn't think so," I said. "Don't remember anything like that last year. I better mention it to Filch, then. Could be a leak."

Honestly, I might have forgotten about the incident altogether if, as soon as we started walking again, the acromantula hadn't whipped around the corner.

"Out of the way!" screamed the beast, favoring two injured legs in its flight and barely making contact with the floor.

Even with the injury, it effortlessly moved onto the wall to avoid us, and continued its dash down the hallway. "It's heading toward the great hall!" Penny gasped.

The three of us pounded after the immense spider, flinging stunners that continued to miss the erratic moving target. Ahead of us, we could spot other kids leaving dinner and I shouted ahead, "Acromantula! Dodge!" Fortunately, they all saw the thing coming and managed to dive out of the way. A mop of red hair that I was pretty sure was Ron Weasley was still safely behind several other kids but shrieked and ran back into the room.

To our great surprise, rather than turning into the dining hall, the spider juked in the opposite direction and crossed the immense antechamber, hit the outer doors with enough force to bump them open so it could get out, and it was gone into the evening.

"What is going on!?" McGonagall asked, rushing out of the great hall with a hand holding her hat on having overheard the commotion.

"An acromantula was in the halls, Professor," Penny explained, slightly winded. "We thought it was attacking, but I guess it was trying to escape outside."

I caught my breath as the professor took that in, and mentioned, "Also, the plumbing might be broken in between here and the library." I thought for a moment, then admitted, "Probably the spider's the bigger problem, though."

We found out later that the acromantula had been the one from our defense class, which Lockhart had forgotten to get back to Hagrid and had just shoved in a corner of the classroom. It had somehow managed to smash its way out of the cage, despite the locking charm. Kettleburn was very impressed. Nobody seemed to be sure what had convinced it to make such an athletic bid to escape.

Filch also didn't find anything wrong with the pipes. If anything, he figured they were cleaner than usual and thought maybe the noise I'd heard was some massive blockage finally breaking loose and scratching down the pipe. I apologized to him that "massive blockage" was a term he had to use in reference to the plumbing, but he said he was used to it. "If anything," he editorialized, "maybe that will give us a little time before that fool ghost manages to make another mess." The ghost that haunted one of the girls' bathrooms was apparently just corporeal enough to back up the toilets.

Sometimes, magic is really gross.