The next morning, When Pomfrey found Severus with a couple of loose teeth and a black eye and so on, she had too many questions to know where to begin, and she opted instead to repeatedly open her mouth to scream at him and then say nothing as she realized he had tried to call her. She had seen a lot, but an attack against a sleeping patient was unprecedented. She made a record in Severus's file, shook her head, paced back and forth, ran two sets of diagnostic spells to check Severus's sanity, wondered aloud who will check hers, and finally addressed him with a direct question: "It does not make sense. How can he become invisible, that's above NEWT level-"

"I don't know how, do I? But he did, I swear he did!"

It was impossible. It had to be a dream. But then, had Severus punched and kneed himself in his sleep? And how did Sirius know about Regulus? And why, why did James and Sirius always punish him for having someone on his side just as he had driven that person away? And why did you think any good could come of associating with a Black? Why wouldn't Sirius's fraternal instincts resurface if it's an excuse to attack you? What did you expect, you idiot?

Pomfrey fixed Severus right up, of course, as soon as she had calmed down enough to focus on her healing spells. He ran his tongue across his teeth and gave her a thumbs up.

He was discharged from the hospital wing for the third time that year, and soon discovered that having an invisible enemy was turning him into a nervous wreck ( even more of a nervous wreck than you've already been , his ever-helpful mind corrected him). He behaved like the complete opposite of his former self – he stayed in groups of people, as much as he could, and he could only feel remotely safe when Sirius was in his field of vision, which was definitely new. He tried to avoid staring at him, knowing that if he could turn invisible, no amount of staring could help. He did not notice Regulus staring at him .

When he was alone, every sound made him nearly curl up in a fetal position.

He constantly asked himself: How? How? How had he managed to render himself invisible, so perfectly invisible, when he was otherwise so astoundingly average? It took the likes of Dumbledore himself to do that. Right, never mind that now – how do you defend against that – Severus could not think, only ruminate.

It was clear that he had finally gone insane. Sirius did not possess such power. You have clearly developed some unheard of magical affliction of the mind. If Sirius knows about Reg, he must have seen you together and simply let it go on, which is impossible… Unless Reg told him? But why would he have, he hates him (Reg hates you too) (but it was not him who attacked you, he does not hate you that much yet) . Severus concluded that the attack had to be a hallucination, and his relationship with Regulus was possibly a hallucination as well. There was only one reasonable course of action.

"Send me to St. Mungo's," he asked Madam Pomfrey.

"Excuse me?" Madam Pomfrey asked, puzzled.

"I have gone mad, and I don't know when it started. Send me to St. Mungo's, they need to sort me."

She sighed. "Why do you think that?"

"Nothing about this year makes any sense. Sirius could not have been here. Regulus could not have been in love with me, I've developed a fixation on that family, I must have, it's the only-" he explained in nearly clinical detachment, convinced that he was making an irrefutable case.

"Stop. That is enough."

Severus stopped.

"I try to stay out of the students' personal lives, but you two have left me no choice. I do not know how Sirius managed to do what he did without getting caught, if it has indeed been him, but Regulus was here, every night, for two weeks, and he was in quite a state over you and what you have done to yourself. You did not hallucinate that."

"Where is he now than?" he asked, trying not to shout.

"It is not my business to know, but he has been here."

"Then I must be hallucinating this conversation," Severus said, defiant.

Madam Pomfrey was exasperated. Philosophical debates with 16 year olds who refused to believe they were not insane were above her pay grade. "That's enough," she said in her most authoritative tone, and Severus's demeanor suddenly changed from defiant to desperate, as he pleaded with her to "get me out of here, just get me out of here, I cannot stay in this place –" he'd almost been reduced to begging again, but Madam Pomfrey acquiesced.

"You can have one night. They'll check you, and hopefully you will return with more respect for my diagnostic methods."

Severus was relieved, hopeful. He was finally going to find out what was wrong with him, what was so wrong with him that he found himself so hopelessly and completely alone, time and time again.

All the while, Regulus stared at Severus trying not to stare at Sirius, and told himself that he will not apologize, he will not break, he will not blink first. If Severus could not see that it was unwise to provoke someone who was already willing to kill him for being an inconvenience, Regulus reckoned it was time to move on. That Severus did not seem to miss him at all did not help things.

To his dismay, Severus checked out of St. Mungo's with confirmation that he was not suffering from any form of insanity known to wizardkind. The professional consensus was that he had indeed been attacked by an invisible Sirius Black.

He had to come to terms with the fact that his one advantage – superior skill – was gone. It was time to invent a new spell, to defend against invisible enemies, but how? And how was he going to test it? He wondered - will he live long enough to get to test it, at this rate?

He borrowed five books about disillusionment and shut himself in an empty room selected precisely for not being the one he and Regulus used to meet at - Sirius knew about that one. If I can't create a new spell, I might as well try to become invisible myself, he told himself. It quickly became apparent that the trick of it was manipulation of light. He would need a spell that worked on entire areas, like his Muffliato, and it could not be too obvious, or he would lose the element of surprise… He racked his brain, when the door creaked and opened, making a shadow across the room, followed by the three-dimensional figure of Regulus Black. Of course, before he could register who it was, Severus was already in a panic.

Regulus raised his hands in resignation. "What's he done now?"

"WHERE IS HE?" Severus shouted at him.

"How would I know?"

"He might be here, Regulus, just go!"

"What are you talking about?"

Knowing full well how ridiculous he must appear, Severus zigged and zagged across the room in random-looking directions to shut the door.

"He can turn invisible, Sirius, and he knows about us, just leave me alone before he gets to you too!"

"Invisible? You're joking, that's-"

"I know it's really advanced!"

Regulus had not seen Severus looking this deranged in a long time. This was clearly not the right time to break things off. He opened his mouth and shut it.

"I'm sorry, all right?" Severus said quietly. "I should not have Confunded him. I wanted to help you, like you've helped me, but I ruined everything. Just go," Severus implored, and this was not out of fear or pride like it was in the dozens of times he had told Regulus to leave him alone before, this was something else, and Regulus could not break things off, although the irony of it had not escaped him.

"Are you going to tell me what happened? Or do you just want to show me?"

"Don't talk about that! He might not know about that yet! Are you sure he is not here?" Severus warned him, sounding more and more hysterical.

"I doubt it, since you are in one piece," Regulus interjected.

"Aren't you listening? He knows about us, he called me a poof, he has seen us, I do not know what he knows!"

"He knows, and yet he did not do anything about it, and did not try to use it against me? And he is suddenly more powerful than Dumbledore? Sirius, who floo-ed himself into Knockturn Alley and almost touched a cursed necklace before we could find him when he was 10? That Sirius? Are you saying he got powerful and patient enough that he would turn invisible, find out about us, and not bash our heads in, if he was here?"

"I know how it sounds. I thought of that. I am not crazy, I checked."

The way Severus said it, it was almost as though he wished he was.

"Right, he can turn invisible," Regulus relented, telling himself this was only for the sake of the argument. "How do you know?"

Severus cast Muffliato on the empty room, not knowing why, and told the story of the attack in the hospital wing.

"... And then I played dead for the rest of the night, and… you know the rest."

Regulus was livid. "I am going to kill him."

"Then this gang will kill you," Severus reminded him. "Don't do anything. I am working on a spell," he continued, as if it was obvious.

"What do you mean, working on a spell?"

"Has he been sapping your brainpower? Is that what has been happening? It means I'm working on a spell," Severus said, and when a look of comprehension failed to appear on Regulus's face, he clarified: "to make it so I can see him."

"You… You can make your own spells?" Regulus asked, incredulous.

"I've made a couple already, why?" Severus answered, taking no notice of Regulus's disbelief.

"Well, do you know how rare that is?" Regulus inquired, more incredulous still.

"Necessity is the mother of invention."

"Excuse me?" Regulus was completely bewildered now.

"Muggle saying."

Regulus was stunned, and Severus resumed pacing around the room and thinking about the spell.

"If I could know how he does it… It's about light, not matter, all right then, light, light, light, Sirius, Black, black, dark, light – shadow!"

He quickly jotted something down.

Regulus looked more and more puzzled.

"He didn't have a shadow!" Severus announced triumphantly.

"So?" Regulus wondered, still not comprehending.

"So, if I could make it so he does, I will know when he is there, I will know where he's coming from!"

"But what difference does it make?"

"It doesn't, really. But I cannot live like this, Regulus. I can't worry about him when I don't see him!"

"But his shadow didn't hit you, did it?" Regulus insisted. "If he is capable of doing that, what difference does it make if he has a shadow? What does it matter if you see him coming?"

"It means I could disarm or slow him down, something, anything-"

"You say you were asleep when he attacked you, you say you cried for help, you say it was about you and me, about us."

"Yes."

"So what difference does it make? You cannot not sleep, and you can't get shot of me, so what does it matter if he's got a bloody shadow?"

"That's not the point, is it?" Severus shouted, shaking. "I know he will stop at nothing, did I not tell you that?"

Regulus was taken aback. "What is the point then?"

"The point is that I thought it was you, and if that's not bad enough, I thought that it was me!"

"I would never do that to y- to anybody!"

"I did not mean it like that."

"And what do you mean you thought it was you?"

Severus lost all patience with him. "It means," he said, slowly at first, then faster and faster as he went, "I thought I'd gone mad, just like you thought, and I have not gone mad, they all insist that I am not mad, so that means I need to worry about being attacked out of thin air, and play dead instead of getting help, and if you cannot understand why I can't live like that, I don't know what else to say, and also, you keep forgetting that he knows about us!"

"All right, all right, I get it!"

Both of them appraised one another, tentatively.

"You are saying you've invented several spells…"

"Will you leave it? Doesn't everybody do it eventually?"

"Well, no…" Regulus said, careful. "I have lived among wizards my whole life, and I've never met anyone who can do that."

"If you say so… but how will I test this one, I cannot make myself invisible."

"Can you make other objects invisible?" Regulus asked, his first bright idea of the day, and a new charm was born: Umbra revelo . The book about disillusionment they had rendered invisible (both of them appreciated the humor) now cast a very peculiar shadow that was more of a reflection, and it was only visible to the caster.

That settled it for Regulus. Severus was never getting shot of him, regardless of what his brother had to say about it.