Snape found himself in his chambers staring at the vial. He'd had a sip, he wasn't hungry, but he needed more. He sat in the small breakfast nook that took up a corner of the sitting room opposite his desk and looked at the vial with a borderline obsessive gaze. He completely understood what was going on, and he didn't like it one bit. Minerva was older, her magic and power were fully mature, and she was a very powerful witch to boot. He ...was addicted, he had to admit that now.

He cursed, anger etched on his face. He had expected this with Dumbledore, but Minerva? No, he hadn't been prepared for this. He should have. He'd seen the magic she could do, he'd thought that since he considered himself her equal in that area that he could dismiss the possibility of addiction. This was apparently not so. He'd been so focused on making sure he wasn't a danger to the students that he had dismissed this possibility from his fellow professor.

He looked at the vial. He took a sip and tried to decide if he cared. He tried to decide what he was going to do about it. He took another sip, and he frowned at his lack of control. It would not do. He had always prided himself on his iron will, and now it appeared nearly shattered.

He stoppered the source of his problems and watched the little bit of red in the bottom swirl. It had been two weeks since he obtained it and the vial was nearly empty. He sighed and sat down, listening to the heartbeats of the sleeping children nearby. They were peaceful, steady, maddening; and that's when he heard it. The louder heartbeat. It was stronger, very strong, and it was moving. It was far too loud for Flitwick, who was the professor on patrol tonight at this hour, in this area. Snape grabbed his wand and bolted out his door looking for, hopefully, an intruder he could eat.

He dashed out of his room and through the hidden door to his potions classroom with a quickly muttered password. Ladles and spoons clanged like chimes in the wind as he practically flew through the room, dodging tables and desks and bursting out the door into the dungeon hallways. The heartbeat was louder, and coming from farther up. He quickly traversed the halls upward. The changing staircases posed no challenge as he leapt between them, ignoring the gasps from three portraits that were awake. He paid them no mind, they would assume it was a leaping spell anyway.

He dashed around a column and followed the heartbeat which was getting louder by the moment. He stopped near a corner as the drum neared deafening levels. Then he smelled it, felt it. Mice, the slight pull. A pull that was steadily growing stronger, exponentially so when he didn't drink from the vial. He exhaled angrily but held up his wand as she rounded the corner.

"Lumos."

The hallway lit up showing McGonagall carrying an armful of books. She jumped back startled. Ready to fight she dropped the books and drew her wand. Then she got a good look at the cause of her surprise and her lips tightened in annoyance.

"Severus! What is the matter with you!? Dashing around in the dark like that?!"

"I could ask you the same." The drum was faster than before. Was she nervous? Or perhaps she was just startled. It was probably the latter.

"I know the castle well enough to not need a light anymore. What are you doing, dashing about like a frantic fox?" She accused as she picked up the books.

"I thought I heard something, it was apparently just a cat with some books." Snape said in an unamused tone as he looked at the titles. "'Bindal's Guide to Vampires', hmmm doing some research on me? Are you scared perhaps?"

Severus held out his arm, letting the professor pass and then walked alongside her.

"Nonsense. I would just like to know how to help you better." Severus could hear her heartbeat hold steady at a fast pace when it should have been slowing, was she lying or just scared? Scared heartbeats always tasted— he pushed the thought away, and the slight heaviness in his head from the red fog receded with it.

"Surely you could just ask me."

"If every organism knew all there was to know about itself it would be a perfect world. That is almost never the case."

"I must agree with you there, but I have read every single one of those books at least twice."

"Irrelevant, I want to read them myself. Besides, I already know what you'd ask of me."

McGonagall waved her hand and opened the door to her office.

"What, pray tell, do you think I'd ask for?"

"More," McGonagall paused as if unsure how to say what came next tactfully "….food."

"I once again am forced to agree. Good night Minerva and good luck with your research." Snape said as he nodded and turned away, as he did he heard her heart slow. Why was she relaxing now? What was she hiding? How could he find out? Would it matter? The heartbeat was so loud, red. He decidedly did not like the feeling it was eliciting in his veins or gut, it was a distraction. Distractions were a chink in his mental armor.

"Good night to you as well, Severus." The professor went to close her door, but found it being held open by a rather strong grip. Snape had a rather unfocused look in his eyes as he gazed down the hall. It was a decidedly odd expression from the usually intense man. "Severus?"

"You should be."

"Excuse me?"

Severus blinked once as if coming out of a daze. He could almost feel that slight crack in his armor growing, this needed to be dealt with. He had not been prepared for this level of assault on his emotions, willpower, or mental faculties. He was sure he could build up a tolerance, could learn to ignore it, but what would happen in the meantime? He needed to think. He looked back at his fellow professor, his face blank once more.

"You should be getting to bed soon. Do not stay up all night. Your students are far more important than my well-being."

"Severus, I'm doing this for them as much as you! Goodnight, you should rest as well."

McGonagall looked at Snape concernedly as she closed the door. Somehow she doubted that was what he was going to say.

Severus stood outside the door for a few minutes. Listening to the powerful drumbeat inside the room, pondering her words; what she, they would do for the students. What he would do for his career as a spy. He had an idea, and would ponder it further, but he had a feeling he would go through with it. He quickly stalked back down to his chambers. He was hungry and that was not good, not for anyone. McGonagall, everyone, should be scared. He would have to make sure that they had no need to be any more scared of him than normal.

Later that evening Minerva got a second alert from the measuring charm. The vial was empty.

Severus spent his following nights in a meticulous order from then on. He graded papers. He drank half a vial of Lintal's blood and obliviated her. He followed McGonagall from the shadows. He experimented as much as he could with his more vampiric skills. And he waited.

He was genial to McGonagall during the day. He was sure he could see her slowly relaxing as time passed. He, on the other hand, could nearly always tell where she was. Always heard the drumbeat when she was near. The longer he went without her blood, the louder it seemed to get. He mused on how long it would take to become deafening, or fade away. He could not risk waiting that long, risk being a danger to the students. He could feel himself slowly losing the ability to… care. It wasn't something that came easily to him in the first place. It had taken years of teaching to develop the ability. It had taken longer to develop the ability to allow himself to care an appropriate amount. Openly caring too much meant the emotion could be used, or it could tarnish his persona. Now, the hungrier he got, the more he felt all of it fade. Felt that chink in his armor grow despite his attempt at occluding. He wouldn't be able to practice, hone, or adapt to this new level of hunger, not while he was addicted.

McGonagall was one person he knew could hold her own against him, and therefore could help. She was also the only one he trusted to actually dispose of him if he truly couldn't be trusted around the students.

Flitwick, a master duelist, was not practiced enough in the art of real warfare. He also lacked the willpower McGonagall did. He would fall. There would be a fight, but he would fall.

Pomona was just in general too kind. She would see him as a friend to be helped instead of a possible danger to the entire castle. That was a mistake she might never recover from.

Hagrid, resistant to magic as he was, might stand a chance if not for his weakness for monsters. Besides, without a wand all he would be able to do was break Snape's bones. They would heal, and that would only leave him hungrier than before.

Sinestra was quite skilled in defensive spells, but he had yet to see her fight, and knew little of her abilities. It would be a risk to rely on her.

Kettleburn was on probation again. While his knowledge would put him at an advantage, the professor was a well known risk taker. He would as likely try to harvest Snape's blood to feed to thestrals to test for unlikely side effects, as to try helping him.

Hooch was highly skilled in matters of defense against the dark arts, and Snape had seen her impediment charms freeze a duelist for a full 5 seconds, but he doubted she would be prepared to both fight him and out-think him.

Besides, he would have to Ask them for help, whereas McGonagall, he suspected, already knew. She was someone that could also, in theory, not upset future plans if the worst happened. Dumbledore, on the impossible chance Snape lost himself and won against the elderly wizard, was irreplacable. Snape would most likely fall to the man, but it was not a chance he was willing to take. He also did not wish for his friend to see him this way, and he did not trust Dumbledore to kill him if it came to that. So, McGonagall it was. The fact that she was also the one he was addicted to… he told himself it was coincidence.

He was right, McGonagall was acutely aware that something was off. Snape had yet to ask for more blood and besides the occasional twitch when he was near her, he was acting normal, which was decidedly not normal in such a predicament. Minerva gave Severus another vial later that week and her concern was proven justified when her measuring charm told her the vial was half empty that very night, and empty the next. She was beginning to have an idea of what was going on.

That night she paced her chambers undecided. If she went to Dumbledore, or anyone for that matter, Snape would be mortified. He would never forgive her. Also, the more people that knew and tried to help, the more people would be in danger. Being around a starving vampire was a risk she didn't want to put upon anyone else, unless needed. However, if she didn't alert someone she or Snape could get seriously hurt. Or the students could. She tapped her chin. The least she could do was utilize the same charm she had on the vial.

Still, taking the evening to mull things over behind numerous wards, would be a good idea. She was sure she would figure out something before she went to bed.

Meanwhile Snape was in the dungeon practicing the Transfiguration Professor's craft, putting in one final night of practice, turning into mist and fog. It had taken weeks, but he had finally managed it. It was the near antithesis of everything he was. Discipline, iron willpower, a constant mask on his emotions and thoughts; he had to let all that go. Becoming mist meant relaxing, accepting that reality and the self were the same, fluid, one river. Luckily, his Occlumency training came in handy. Allowing the Dark Lord to peruse his mind meant his thoughts had to be fluid, able to be turned in a different direction at a moment's notice. Hidden, compartmentalized. If he could let his mind be fluid then he could pretend he was relaxed while still keeping control of some aspects which he could not let his guard down around.

His hunger and addiction brought the red mist from the edges of his mind, to tell him horrible things. Things like he could have easily killed young Banderknott and disposed of the body in the forest, near the acromantulas. Things like a few bottles of elf made wine, and Sybil's broken body beneath her window, with a blood replenishing potion shoved down her throat before she died and no one would be the wiser. He could not relax and let those thoughts become commonplace, they were not imaginings he cared for. So he had to compartmentalize, relax as much as he could, while pretending he was just Snape, a professor, and nothing else. It made sense that being something so ephemeral required one to relax, still, it had been a process to get there. And once there, one had to remember who they were or risk fading into the ether. Still, it was worth it. The long nights spent compartmentalizing for as long as he could, until he sweated tiny beads of red, until he could fake relaxation.

Being in such a form meant many spells would be ineffective against him, and he had to admit, it was rather intriguing.

Both Snape and McGonagall headed to bed that night with machinations and decisions finalized.

The next day McGonagall came to Snape with some papers.

"Hello Severus. There is a Gryffindor student who needs to be excused from potions until he can get his anti-allergy charms redone. There was a misunderstanding about appointments at St. Mungo's and he will have to miss your class for a bit. This requires your signature."

"Very well Minerva. Give me a moment." Snape scrawled his name quickly across the bottom and handed the parchment back. Before he could say anything else McGonagall had turned the proverbial tail and left. Snape stared after her and listened to a drum only he could hear as she left his office.

Back in her quarters the professor lifted the illusion on the top part of the paper and nodded, satisfied. After filing it away she headed to her bedroom and set about the next part of her plan.

That evening Snape stalked the corridors as usual, passing the Slytherin dorms first, then up to Ravenclaw, down to Hufflepuff, and finally toward Gryffindor. He made the circuit twice, before deviating and heading toward McGonagall's office on the first floor. He had divined the location of her rooms from where her heartbeat stayed every night. It had not been hard. One does not sleep in their office after all. Not every night, anyway. Not even the studious McGonagall.

He unlocked her office door with a few spells and headed in. It was sparse, but the tapestry on the back wall was what drew his interest. The drumbeat behind it was steady; his quarry was asleep. Another small beat pattered away in the hidden room, perhaps Ms. Norris had chosen to sleep with a fellow cat. He'd take care of the cat if it woke up. Preferably non-lethally, but cats had never been fond of him.

With another deep breath he relaxed and willed himself beneath the transfigured door, turning into a fine mist and wafting between the cracks in the stone. The world seemed far away as he melted, but he kept his goal in mind and moved forward. As he coalesced on the other side he could feel the slight tingle of a trapping charm that hadn't activated due to his miasmic state. He mentally smiled, his quarry was smart, but not smart enough. He flowed forward and reformed beside the bed from whence the drumbeat emanated. The comforter was a bright red with a gold feather motif. He held back a sneer at the predictability.

McGonagall was completely under her covers, he quickly peeled back the top to reveal...nothing. A knot grew in his stomach as he pulled the blanket back further still, to reveal a drum, a beating charm causing it to reverberate rhythmically. He turned to flee but too late; a field of force had surrounded him on all sides as soon as the drum was revealed. He stopped as he heard a rustling of blankets from the other side.

In the corner a cat with square markings around the eyes wiggled out from under a blanket. It stepped out from the cat bed and took another step, into McGonagall's form. She stood dressed in a nightgown, with wand at her side. She looked at Severus who returned her gaze with one of utter loathing tinged with respect. Minerva shook her head.

"I am truly sorry Severus, this is all my fault." And with a quick stupefy that was all the Potions Master knew.