Bob, my skull-bound assistant, was a powerful magical library that ran on smut, and it took me until a week after school was back in session to find a new source and an opportunity to have a private conversation.

"Harry, these are nasty. I love it. Where did you find these?" chattered the skull, happily.

"The school librarian has a huge collection of risque romance novels," I explained. I was half thinking about telling the twins about Bob, because the reasons they imagined I'd wanted them to lift a few of the books were probably going to get me in trouble some day.

"Well, she has excellent taste." A skull shouldn't be able to leer, and yet. "What are we doing tonight?"

With a bunch of effort drawing runes and circles in chalk, I'd put some silencing and locking charms on the spare classroom I'd set up in, and hoped any school officials would buy that I didn't want my enchanting interrupted rather than that I didn't want anyone to see me talking to an arguably dark artifact. "Working on a transfiguration focus and talking about magic resistant creatures."

"Transfiguration foci are hard," Bob admitted. "It's one of the advantages of a wand. Each transfiguration is so different, it's hard to put in a generic matrix. What do you want to transfigure?"

"Ran into some leucrotta last week, and needed iron spikes to fight them. Wouldn't have been able to do it fast enough without help from one of the other students."

"Leucrotta. Hate those guys. Won't shut up. I see why you want to talk about creatures. I mean, if all you wanted was something to turn out spikes, that might be doable. But it would probably be pretty limited about what you could start with. Why don't you just carry some spikes?"

I frowned at how limited a focus was going to be for this. "I'd really hoped for something small that would get me a bunch of different offensive materials. Iron for fae, salt for spirits, maybe some lead shot for other things. I don't think I should just stuff my pockets full of that kind of thing while wandering around a school."

"Utility belt!" exclaimed the skull.

"You read Batman?" I asked, surprised based on his normal reading material.

"Poison Ivy and Catwoman," he said, as if that explained everything, and maybe it did.

"Anyway, I don't think I could carry a reasonable amount on a bunch of pouches without it being really obvious and heavy. And too many extension charms that close together shouldn't be arithmantically possible…"

"Maybe for normal wizard math," he scoffed. "You just have to make sure you stagger the extra-dimensional direction each one points so they don't overlap, and anchor it all in a very stable material with some failsafes." He looked at my pile of enchanting supplies, including the hides Hagrid had given me for Christmas. "Is that thestral hide over there? That will be perfect."

While it wasn't remotely what I'd come in expecting to do for my next project, Bob was the expert and I couldn't fault his logic. So I spent a while copying out the rune and matrix logic I'd need to enchant a thestral-skin belt to hold way more than it ought to. It was much more elegant than what I'd done on my own with my charmed bag.

Finally approving my draft of the project, Bob asked, "So you fought leucrotta at Hogwarts? Not supposed to be many of those outside of India."

"That's what Hagrid and Kettleburn thought," I admitted. "We had a troll get let into the castle a few months ago, and someone gave it a student's sweater to try to go after her. I don't know if the same person got some leucrotta. It went after one of her friends."

"Can't convince a leucrotta to do anything but be a jerk," Bob disagreed. "Try to ask, compel, or trick one into doing what you want and it will do the opposite just to spite you. They will go after kids and dogs without prompting, though. Think they're delicious."

"I guess someone still could have let them onto the grounds, knowing Ron was going to be the smallest kid around over the break and liked to play outside…" I sounded dubious.

"But you thought to use iron to attack it for a reason," Bob finished.

"The professors did think they looked different than ones you normally see, even in India," I sighed, admitting to my hunch. "Damnit, Bob, it was as big as a moose. Nothing that big is supposed to be able to get out of the Nevernever."

"Veil isn't what it used to be," he mused. "Wizards have been using it as a highway for centuries, and too much space-warping doesn't help it either."

"Says the guy who just taught me to cram a dozen extension charms into one belt," I snarked.

"With the Nevernever footprint this castle has, a utility belt is barely going to register," he disagreed. "Still, it is weird that multiple big things got out. I wonder if anyone's messing with the raths."

"What?" I gulped.

"If you apparate from one of the old 'faerie mounds' you can go further, easier. But it's kind of like a slingshot. You stretch the Veil so much to launch yourself that it can temporarily warp enough that opportunistic beasts can get through."

"That bitch," I said, angry and guilty. "It's my fault. My godmother told me how to do that. Even gave me a map of where to find all the raths in Britain. I used the one at Hogsmeade to get down to London to pick you up."

"Huh. Well, yeah. That would do it." I thought the largely amoral skull was going to leave it at that, but he added, "I wouldn't beat yourself up, though. She can probably do the same thing whenever she wants, so it's not like she tricked you into doing something she couldn't."

"Maybe," I sulked, worried I'd unleashed faerie predators on the school and wondering if Bellatrix had intended for me to do that. I also needed to figure out how to warn Percy, Penny, and Oliver not to take advantage of the raths without admitting that I was inadvertently responsible for putting Percy and his brothers in danger.

"Anyway. Once you have pockets full of Kryptonite, is that all you needed to know about magic-resistant creatures?"

"Was that another Batman reference?" I boggled.

"Well, that, and some of the workers at the post office were playing the new Spin Doctors album and I was bored stuck in my box. I know all the lyrics. Want to hear?"

"No thanks," I told him, not totally sure what he was even talking about but sure his singing voice wasn't a talent. "And, actually, I had another question about magic resistance that we've been banging our heads against. What do you know about the vulnerabilities of dementors and other wraiths?"

"Your premise is faulty," he explained. "Dementors aren't wraiths. They're corporeal and soulless. Admittedly, they're basically undead lethifolds with a bunch of other dark magic worked into them, but they're not wraiths. Those are always incorporeal and all they are is souls."

"So it's just a ghost?"

"Not really. Ghosts are more like a shadow of a soul left in the world by your magic. The soul moves on, but if you had unfinished business your magic basically sears an imprint of you into the world. Ghosts can't really change, make long-term plans, or get over what they were like when they died. And they can't possess a body."

I blanched, thinking about the thing that Justin had summoned. "Possess a body?"

"It takes a lot of dark magic and sheer bloody-mindedness to become a wraith, Harry. The mechanisms for making sure mortal souls leave the world when you die are core components of reality. You're wearing your mom's necklace. Think about how one of the most powerful legendary artifacts just lets you talk to the departed, and explicitly can't bring the dead back to life. People that become wraiths aren't just looking to float around for eternity, they're usually trying to claw their way back into the world. The ones that don't want to steal a new body are scarier than the ones that do. Tolkien knew what he was talking about: there have been things like the Nazgul throughout history and they were terrible."

"Is there a way to fight them?"

"Well… actually… remember how I said that dementors weren't wraiths? Let's talk about how the patronus charm works…"

"Soul magic," I said, dropping an ancient tome on the library table in front of Percy and Penny. It had taken me a few days after Bob had put me on the right track to find a useful reference in the school library. I wasn't about to tell them about the original provider of my information, so I needed a more acceptable source of the details.

"Summoning Angels," Percy read from the title page of the book bound in long-yellowed ivory leather. "Did you find this in the historical fiction section, Harry?"

"Yeah," Penny added, flipping through the ancient vellum pages of the large book, "I've never heard of a witch or wizard who believed in angels and didn't get it from muggle religion."

"Two good reasons for that," I allowed, because I'd had the same questions for Bob. "Whether or not anything about it was true, the wizarding world soured on mainstream religions really hard during the witch hunts. And the angels had stopped showing up long before that because the goetic wizards kept trying to bind them like demons." I gave it a beat. "Yes, there are also demons. Nogtails, grindylows, and some other magic beasts are descended from earthbound ones."

They flipped through the book for a few more minutes, studying for OWLs temporarily forgotten. It was beautifully illustrated, even if the handwritten Middle English text was difficult to parse. Finally, Percy admitted, "This does look authoritative. I need to do more research. So what is this about soul magic?"

I wondered if I should have prepared visual aids, or at least had some paper to draw on, as I started to explain, "According to this book, if an angel wanted to grant favor to a wizard, it could provide an ability to form magic directly from that wizard's soul. It was basically the opposite of demonic bargains to channel energy from the fires of hell. The book's name for it was dumb, so I've been thinking of it as soulfire, since it's the opposite of hellfire."

They'd found the page I marked about it, and Penny started to slowly translate the book out loud, "And amongst the powers of these arts came conjurations of great strength, invulnerable wards, and," her voice rose in excitement, "the ability to harm those thought unassailably steeped in darkness."

"That is a better reference than our previous efforts, Percy admitted. "But if angels no longer grant favors to wizards…"

I nodded. "If we could somehow get imbued with soulfire, we wouldn't even need a spell to harm wraiths and dementors, just put it into any other offensive spell. But that's not likely to happen. However," I flipped to the second section I marked, "there is one known spell that uses soul magic."

"Yes! We were on the right track all along!" Penny exclaimed, seeing what was clearly a description of a prototype of the patronus charm.

"So the patronus uses energy from your soul, not just happy thoughts?" Percy asked, then clearly thought of something. "That would actually explain why dark wizards have a hard time casting it, even if they have something to be very happy about. Dark magic is supposed to be damaging to the caster's soul."

I nodded, frowning. "What Quirrell left out in his lesson about Unforgivables is that using magic to kill, compel, and cause pain can cause wounds and fractures in your soul. Even killing or torturing someone with non-dark spells, like the fire-making charm, can cause problems. The Unforgivables are so focused on what they do it makes it worse."

I lapsed into musing about how wounded my soul must be having used a dark spell to kill Justin, even if I hadn't meant to. That gave them time to think it through for a couple more minutes before Penny spoke up. "We're on the right track, but the patronus just creates a protector. It's more like what the book said about making strong conjurations and wards. It's not meant to be offensive. At best, I've heard a strong patronus can shove a dementor around. I don't know if it's enough to come up with an offensive spell."

"That's where I'm stuck too," I admitted.

Percy was looking a little inspired, though. "Hellfire is the opposite of soulfire, right? I recall a book suggesting that the earliest dark magic was taught by demons. What if dark curses are based on demonic magic the same way the patronus is based on soul magic?"

"That's brilliant, Percy!" exclaimed the Ravenclaw prefect, not noticing that she'd grabbed his arm in excitement and that he'd blushed as red as his hair. "If we could diagram the arithmancy of the patronus and compare it to a dark spell with a similar enough effect, we could see if there are parts that are the instructions for using, well, light instead of darkness. Then replace those parts in an offensive dark spell."

"Fiendfyre," I said, thinking of the bestial fire construct it had summoned, so similar to the protector imago of the patronus.

Percy managed to focus and allowed, "No protective component, but we might not find a protective dark spell, let alone one that is at all similar. The problem is that the spell is highly restricted. Maybe the headmaster will let us into the restricted section if we explain what we want to learn it for…"

"We won't have to go the restricted section," I admitted, "I already know how to cast it."