Dudley opened his mouth, a long "oh" sound coming out. Then, he asked, a little diffidently, "Why blood, though? I could just clip a fingernail, couldn't I?"
Draco answered that one before Snape could start in with big words for bigger concepts. "Blood's actually better. It's a powerful magical force, which explains why what Harry's mum did for him is called a blood-sacrifice, see? Besides, all we need is a couple of drops."
Dudley measurably relaxed. "Oh, all right. Why didn't he say so? I can do that."
"Good," Draco approved, beaming an encouraging smile. Perfect teeth, Harry thought, then immediately discounted that as more Slytherin cheating. Magic braces, something like that. "So, are you ready then? We won't need the blood for a few minutes. First Severus has to do the incantations and draw power from the air and stuff like that. Then he'll ask you some questions, and then, we'll seal the warding with your blood, and voil‡, it'll be done."
"Questions?"
"Yeah, they're sort of like vows. You have to agree to all this to make the magic work."
"I get to make the magic work," Dudley marveled, his eyes sparkling a bit.
"Yeah, we couldn't do this without you, Dudley," Harry put in, nodding. "So thanks. It means a lot. With the spells in place, nobody who means me ill will be able to get into these rooms."
"It's all just a way for me to make Severus let me live in Slytherin," Draco joked, but his silver eyes were wary as he watched Harry take in the comment.
Harry glanced at him, but said nothing in reply.
"Well, even if she didn't have to... er, bleed, I still can't really see Mum letting wizards into her house to wave wands all around."
"No, she wouldn't have," Harry agreed. "All she had to do was take me in. But she was closer to the original power of my Mum dying for me, Dudley. I mean, she knew her sister, right? Grew up with her, all that. So for her, the transfer was sort of natural. Professor Snape has to do more formal magic to transfer my Mum's blood sacrifice to you. It's pretty complex stuff."
"I think you offended Severus," Draco said in a stage whisper. "It's interaxial multidimensional sorcery he's about to perform, not complex stuff."
"No wand waving or silly incantations?" Harry laughed.
"Be quiet, both of you," Snape instructed. "Just watch. Maybe you'll learn something. Dudley. I need you to stand next to me."
And so it began. Harry stood up from the table and backed away as he watched Snape begin both wand waving and incantation, but none of it was silly. He chanted rhythmically in a language that sounded somehow Latin, and yet older than that as he pointed his wand at all the corners of the room. Silver threads formed from his wand and spun out to those corners. The threads wove themselves into a shimmering spectral fabric that began to coat the walls.
Draco pulled Harry away from the granite before the shining tapestry touched him. Harry couldn't help what happened next. He flinched violently away, stumbling so severely that his feet slipped out from under him. He landed on his arse, his skin feeling like it had been doused with boiling oil, even though Draco had only touched sleeve, not skin.
He looked up, only to see that Draco looked absolutely ashen, his silver eyes haunted.
Harry remembered then, what Snape had said, that Draco was quite literally terrified that he could someday be thrown to the wolves on Harry's say-so. Harry certainly didn't trust the Slytherin boy... not even close... but he didn't want Draco thinking he'd lurched away because of that. It was more that he'd been startled. Even Snape's hand on his shoulder could disconcert him if he wasn't expecting it.
Harry couldn't explain all that without speaking and disrupting the spells being cast, so he did what he could. Biting his lips to hide his grimace, he extended his hand towards Draco.
The Slytherin boy's eyebrows rose, and for a moment he just stared, but then he helped Harry up.
Snape began walking, continuing his chant, entering every room and spelling it in the same way, one hand on Dudley's elbow keeping the Muggle boy with him all the while. Following along, eyebrows raised, Harry noticed that Snape warded the ceilings and floors, too, the silver shimmer of the phantom tapestry acquiring an aura of gold as it continued to weave itself thicker.
When the entirety of Snape's quarters were coated in the stuff, all of them stepping in it despite Draco's earlier caution, the Potions Master fell to his knees and incanted one last spell.
Instantly, all the warding flew back towards his wand to coalesce into a glowing ball floating in the air above Snape's outstretched hand.
"Dudley Dursley," he said, the English sounding harsh after all those soft Latinate sounds, "do you give consent that this domicile may host the powers that will protect and preserve your mother's sister's son, Harry James Potter?"
"Yes," Dudley whispered, looking sort of appalled, of all things. Harry figured that just came from him never having seen any ritual magic before. Draco's dancing candies definitely didn't count.
"Do you consent to yield up blood such that his mother's love-sacrifice may continue to reside in this place?"
"Yes," Dudley said again, and that time, he just looked plain scared. Probably the mention of the blood.
"Harry," Snape said, prompting him. They'd discussed this. It was taking all Snape's power just to hold the warding spells in place for the blood binding. That pulsating sphere above his hand was made of spells. Snape couldn't both keep it coalesced, and drip the blood atop it.
Stepping close, Harry took the ceremonial blade Draco held out, and with an apologetic wince, made a tiny slash in Dudley's palm. Holding his cousin firmly by the wrist--and ignoring the tremors that caused him--he turned the palm to face the floor and let the blood drip onto the warding spells Snape was holding steady.
Instead of being absorbed and made a part of the magic, as they all expected, the blood fell straight through the luminous sphere to drip onto Snape's own palm.
And then the concentrated magic in that sphere wavered, the ball undulating, unraveling, and vanishing from existence.
Snape uttered a long, low curse, and shakily pushed himself to his feet.
Draco stopped breathing.
Dudley rubbed his sore palm and looking around, said, "Is that it, then?"
It was left to Harry to conclude out loud, "I think it didn't work."
"No, it didn't," Snape concurred, his tone rather bleak. "The physical manifestation of the spells should have turned the color of blood, and then, the color of your mother's love, and flown back out to melt into the very stones that comprise these rooms."
"What went wrong?" Harry pressed.
Snape didn't answer that. "Draco, Floo the kitchens for something light to eat and drink." He sat at the dining table and beckoned everyone to join him, shaking his head at all questions until he'd drunk a full cup of tea and eaten a couple of finger sandwiches. Then he observed, "The form of the incantations was definitely not the problem. I think it must lie in the applicability of the spell."
"You said Dudley's blood could only be used to ward a personal residence," Harry remembered from their conversation in the hospital. "Maybe this doesn't count as one, even if you have been the sole occupant for years and years?"
Snape shook his head. "I did some other spells to check for that. The rooms here believe I'm the owner."
"Then that's the problem," Draco pronounced, waving a hand toward Dudley. "He should be the owner, surely, if he's going to be the key to warding this place. We're looking for things to be parallel, right? Harry's aunt owned her house, after all."