Although he didn't know it, and he never would, once, before Harry Potter was ever known as the "Boy Who Lived", the "Chosen One", Severus Snape had made a promise to his mother, a promise that he kept long after she was gone, a promise that he would keep for as long as he had to.

"I will keep you safe," he had said, taking her hands in his, refusing to let her look anywhere but his eyes. "Forever."

She had laughed at him then, Lily, her laugh like a delicate spoon against crystal. "I don't need protecting," she had told him, words that would echo in his ears even decades later. And then, softer, "but thank you."

He had loved her, more than anyone had ever loved anyone else in the universe, he was fairly certain—more than his father had loved his mother, to be certain, and more than even James, her husband, loved her—had never stopped loving her, and by extension, in some small, strange way, he loved her son, her insolent, overly confident son, a perfect mirror of his father in look and temperament, because he was the only part of her that Severus had left, and he couldn't give up on her. Not now. Not yet.

Countless times the boy had walked into death's alluring traps, countless times challenged—no,practically taunted death to take him, to make the Boy Who Lived the Boy Who Lived No Longer, and countless times Severus had stepped in, saved his life, without a word from either of them, some unspoken agreement that Harry didn't even realise had been made.

And he had tried to keep Lily safe, too—had been successful, for a time—used his insider information on both sides to let her cheat death over and over until they could cheat no longer, until, as with all things in Severus's life, fate caught up with him.

He had tried to save her the night that she died. It would be Voldemort's fourth attempt that year at attacking the Potter house, the previous occasions having fallen through on various security issues, and his bloodlust was unfathomable, stronger than it had ever been. The Potters needed to die, he said, for they were the greatest threat to the uprising of the Dark Lord, the only thing, he believed, standing between him and true domination. And so for a week prior there would be a 24 hour watch on the Potter house to make certain there would be no escape, and a date was set—Halloween, shortly after nightfall. He would go in alone, with Yaxley and another Death Eater—a volunteer—standing by in an abandoned building nearby in case backup was needed

Severus had volunteered himself, of course, his mind frantically formulating a plan even as he sat there listening to the Dark Lord speak. He would arrive early—very early, perhaps as early as the day before, while James was at work and Lily was at home with the baby. He would tell her of Voldemort's plan, and together, with the boy, they would flee, leave the country and go into hiding, start a new life, a life together far from the threat of dark wizardry and, perhaps worse, of James' wrath But there must have been something about his eagerness that tipped Voldemort off that his intentions were perhaps not entirely aligned with Voldemort's own, and it was decided that another would go in his place.

It was then that Severus knew he would have to break his promise to Lily. There was no chance for escape, the house was surrounded and Severus was no part of it. There was nothing left for him to do now, nothing left but to sit back and wait for his love to die, for he had, once again, ruined the only thing that he had left, and with nothing left, he retreated into self-imposed exile, hiding himself away in his home so that he could mourn in his own way, without disturbance or dramatics.

It wouldn't be for several days thereafter that Severus would finally hear word of what exactly had transpired at the Potter house on that night, until he finally emerged from his hiding place only to be greeted with Lily's—and the baby's—smiling faces on the front page of the Daily Prophet. "The Boy Who Lived!" declared the headline. "Could an infant have defeated the most powerful dark wizard of our time?" Severus did not need to read the article to know that although he had failed to keep Lily safe, he did not have to break his promise. Somehow—call it chance, call it miracle, call it a disastrous failure, depending on which side you were on—somehow, her son had survived, a part of her lived on, and Severus needed to protect him, for his mother's sake. He would never know why, never know what it was that Severus Snape saw in him, but he would always be kept safe. He would not, as long as Severus could help it, fall to any harm. He was the last piece of Lily that Severus had left, and he couldn't let her down again.

And now they were at the end of their journey, he and Harry. He had done all he could, and Harry had survived all manner of unspeakable danger, had fought so hard for so long, and although Severus knew that he would not be able to protect him any longer, knew that the end of his life was coming in short order, he also knew that he had done his job, had kept his promise, and now Harry's life was in his own hands, no longer in need of protection.

Severus wished now that he could tell Harry how proud he was of him, how much he loved his mother, loved him, how although he had protected him for so long he knew he could do it on his own, but he knew there wasn't time, and all he could do was hope that on some level, somewhere within the deep reaches of Harry's subconscious he knew, that what Severus had left behind for him would be enough to explain it further, reach out for Harry's hand. "Look at me," he demanded of him, though his voice sounded more weak and less authoritative leaving his mouth as it did in his head. "Look at me." And Harry had looked down at him, more confused than concerned, but they were Lily's eyes all the same, and Severus struggled to match his gaze for several seconds before his body was filled with the warm relaxation of death. He had fulfilled his promise and now he could die happy, could, after nearly twenty years of waiting, see his love once more.