Bonds are fickle things, they say. They don't form immediately, but from repeated exposure. There is time to reject them along the way, but what if fate wishes otherwise?
"Severus! I need you!" Poppy bellows through the Floo, startling him. "Come to the Infirmary at once, bring more Blood-Replenisher and Pain potion."
Abandoning the stack of third-year essays on his desk, which he has stared at unseeingly ever since returning from Flooing Grimmauld to check on Black who unfortunately was right where he was supposed to be, Severus hastily summons a couple of vials and heads for the Infirmary.
The Infirmary is in chaos, more than usual. A few Potions accidents during Horace's time came close, and one memorable Hufflepuff-Gryffindor Quidditch match in the mid-80s where three players were taken out by Bludgers and the onlookers all started hexing each other. This time, students are being brought in on stretchers by Order members. Other beds are occupied by students after the DADA exam and what appears to be the illustrious Head Inquisitor herself, twitching on a bed with a curtain half askew, clothes in disarray and twigs in her hair. Lupin is on his knees by the Floo, talking to someone. People are bustling through the room, looking busy, trying to make themselves useful but mostly just being in the way. Molly and Arthur are easily recognisable by the hair and by Molly's high-pitched wail. Tonks is there too, her hair dimmed to a dull brown, looking quite a bit worse for wear.
"Severus! Can you please check Miss Granger?" Poppy shouts over the din while examining Miss Weasley.
Miss Granger is on a bed by the window and appears stable at a first glance, Poppy having bandaged her chest. She is either unconscious or sleeping, but probably the former. She's pale though, her lips turning blue. There is something dripping on the floor from the middle of the bed, something dark.
Severus rips off the sheet covering her. "Poppy!"
The matron runs to him while he is Summoning more potions from his stores. Blood is gushing down on the floor from the wound that stretches from Granger's clavicle down across her chest to the opposite hip. The edges are dark and apparently trying to expand, like ink soaking into tissue paper, while black bubbles are forming in the middle.
He hands the vials to Poppy and starts casting counters to the Dark magic that is currently hissing malevolently in her chest, while Poppy works on the edges of the wound and makes sure the potions end up where they should.
"There's something else there?" Poppy asks. Her wand is weaving intricate patterns over her charge, checking diagnostics and healing other wounds.
Severus nods curtly, still chanting counters. Dark magic is insidious and innovative. It's gained a foothold in her flesh and doesn't want to let go. If he knew who the caster is he might be able to counter it better, but he doesn't, so he cannot.
"Think we've stabilised it enough?"
He shrugs. No, they haven't, but Poppy clearly hopes for time and needs to save everyone else. She's usually level-headed around injuries but Dark magic is not common in a school, even a magical one. He's seen these kinds of wounds before, though, and there's nothing Poppy can do.
"I will need to brew another antidote. She's not in the clear yet."
He goes down to the lab again, setting up a workstation to brew another painkiller and a specialised Healing potion. Chopping, dicing and grinding ingredients, his hands working by themselves while his mind is whirring with options and things to test. Most of them won't work, however.
It had been a long week already before today happened, with Quidditch accidents and several exam-induced injuries and overwrought students. The illustrious Head Inquisitor hasn't made his life easier, either, and he wonders idly who might caused her to end up on a bed in the Infirmary looking like she'd been dragged through the Forest. He might even consider awarding them House points.
Potter's abysmal attempt at subtlety should have fooled exactly no one apart from apparently said Head Inquisitor and perhaps Goyle on a bad day, but apparently Potter thought even less of Severus than he'd have guessed since the boy immediately ran off to London, of all places. And for what? Getting his friends hurt, or worse? Severus definitely needed to get the full story from the Headmaster or whoever else had information. Black had been his usual unpleasant self at Grimmauld. The students had clearly been cornered by Death Eaters, but why and how? It doesn't make sense. None of it makes sense, and Severus hates things not making sense.
The potions bubble and hiss in their cauldrons. He checks the consistency of the Healing potion and adds another ounce of diced Flobberworm before lowering the heat to let it simmer.
He returns to the Infirmary late at night, it must be past three in the morning already. Poppy is asleep on one of the beds, dozing fitfully while her monitoring charms and wards watch over the students.
Granger is not recovering. She's pale and sweating and when he pulls the blanket back the wound has started to grow again, looking almost as bad as when she first came in. Black tendrils branch out from the edges, trying to gain ground. It's a wonder her status hasn't triggered the monitoring charms. He silences them temporarily, no point waking up Poppy if there is nothing she can do for her patient. She wouldn't agree, he knows, which is another reason to do it.
He swears again, reverting back to the language his Da was so inventive with, Northern accent returning as well while he pours the vials down her throat. Pain reliever, a healing potion strong enough to cure a Hippogriff, and another of his own invention designed to counter Dark magic.
They don't help. Another diagnostic charm shows everything heading in the wrong direction. Hermione Jean Granger is dying, a teenage girl who hasn't even finished her schooling, the one bright spot of his teaching career about to leave him forever.
Muttering a curse he remembers one more thing he could try, a last-ditch attempt with rather severe side effects. Last-ditch because it is considered borderline Dark, but also because of all the ways it could go horribly wrong. He doesn't have time to verify their blood type compatibility, he thinks he recalls seeing it in Poppy's more extensive diagnostic charms but he doesn't want to wake the school matron unnecessarily. If this works, it works; if not she will die anyway.
"Needs must," he mutters as he makes a precise cut in his right wrist with a carefully aimed spell. Blood starts welling up immediately. Cursing again at his own lack of preparedness he Transfigures a cotton swab to what he needs, a long plastic tube with a needle at each end. He thinks Granger would probably appreciate the Muggle tech; St Mungo's would never have come up with such a gadget. It is difficult with the pain in his wrist, but he has cast spells through worse, before.
After inserting the needle into his vein he plugs the other end directly into a large vein in Miss Granger's arm, and holds his arm above hers. In theory, since he has survived Dark magic of a similar kind to what is now trying to kill its young host, his blood should carry sufficient magical antibodies, as it were, to help her own body fight off the invasion. The Dark Mark complicates matters, but he doesn't think it will infect her.
He feels his blood draining, seeing it enter her. As he grows weaker he can see colour returning to her face. Is it working? He needs to keep it up, to give her a chance to live. Cursing again he realises there isn't any Blood Replenisher nearby and his magic weakens at a similar rate to the blood loss, so he can't summon one while his wand arm is pierced. He holds on to the bed and then has to sit down clumsily on the edge of it, trying to maintain the transfer. He really, really should have planned this better.
Just as he thinks he is about to pass out he sees her eyes flutter under her eyelids, and she takes a deep breath. He yanks the needle out and seals the wound on his own arm with a bit of gauze, too weak for spells, holding it in place with his left hand. At least he can wield his wand again, letting him Summon a Blood-Replenisher for himself. As usual the taste is horrible, it isn't the special version he brews for himself with a more neutral taste, but the bulk version he brews for the Infirmary. Still, it is working, letting him regain enough strength to Heal the wound on her arm and Vanish the evidence, including the needle and tube.
"Prof..." she whispers, eyelids half open.
"Sleep," he mutters, suddenly feeling completely exhausted himself.
Feeling ever so grateful for the Floo system he returns to his chambers and crashes into bed, only barely managing to Divesto his robes and boots first.
The next morning he drags himself back to the Infirmary after requesting an extra large cup of coffee from the elves. Breakfast hour has already passed, but there aren't any other demands on his time.
"It's a miracle, Severus!" Poppy looks a lot happier this morning. "She must have thrown off the curse during the night."
"A miracle, Poppy?"
Poppy gestures at their patient who is propped up with pillows to a half seated position. Her skin colour is back to normal, perhaps a tad paler than usual but otherwise perfectly normal, with no signs of what she had been through over the past twenty-four hours.
"Yes, look! She's fine. The scar is almost gone, too."
Brown eyes follow his every move from the bed, but she doesn't say anything. She raises an eyebrow at him. It's almost like looking in a mirror, his expression in her face, looking impassively back at him.