"A blood ward," Quirrell said to the class, stuttering even more than usual, "is a powerful form of magic. It can be used to protect someone against almost anything, but it requires an ultimate sacrifice of someone with which you share blood." He pulled a tray of small vials from a cupboard. "The act of bloodshed has a long history in magic. While creating a blood ward requires a person to give their life for another, limited protection spells can be created by much less final means, as long as the person shares blood. Today, we are going to find out who in this room has enough common blood to create a blood talisman for each other." He held up a tiny glass bulb filled with red liquid on a string. "Some of you may leave this room more safe than you were when you entered."

With a wave of his wand, the vials flew out of the tray and landed lightly in front of each student. "There is a spell," he said as the vials sorted themselves, "that allows you to puncture skin enough to draw blood. But I was told that I am not allowed to teach spells that draw blood to First Years. So instead, we will have to embrace the archaic Muggle method. Needles." With the same wave, small needles shot across the room to each Gryffindor. "These needles are steel, not silver, so the vampires and werewolves in the room can remain unconcerned."

Olivia wasn't really sure if the man was trying to be funny. His comedic timing was certainly a bit off, due to the stuttering. But his cadence resembled Severus's when he thought he was being funny.

She took the needle and stabbed it into her finger. Next to her, Hermione winced and then watched the blood drop from Olivia's finger with distaste. "Is this really necessary?"

"Really? This is where you draw the line?" Olivia laughed. "You will do anything for a grade. You'll stay up all night, you'll steal books from the Restricted Section, you'll interrupt my dad to ask a question, which is basically suicide, but poking yourself in the finger is too much?"

"Hurry now," Quirrell encouraged. "We don't want some people's blood drying while others are trying to get to the point, as one might say."

Olivia rolled her eyes. This was Ravenclaw humor for sure. She watched with interest as Hermione finally pricked her finger. Once everyone had done so, Quirrell raised his wand. "Colligationis," he said, flicking his wand at the class.

The vials started to glow different colors.

"Look around the room," Quirrell said, walking down the center aisle. "If your vial matches another vial in color, it means you share lineage. The brighter the color, the closer the match."

Olivia barely bothered to look around the room. She was more likely to share blood with a classroom of Slytherins. Her vial was glowing a deep red. Glancing down the row, she saw Ron's vial glowing red as well, although less vibrantly. That wasn't surprising. All the pure blooded families were related somewhere along the line.

Next to her, Hermione's vial was a pale periwinkle, not matching anyone else in the room. Then her gaze fell on Harry's vial. It glowed deep red. An exact match to her own.

Severus strode down the hall, robes billowing behind him. Cleaning up memories after the Defense lesson had been a trial, and every minute it took, he dreaded dealing with the student in his living room. It was troubling that Quirrell had introduced such a lesson to begin with. Blood wards were not First Year magic to be sure. If his suspicions about Quirrell's alliances were correct, this lesson was likely an exercise in discovering how many people at Hogwarts shared genetic ties to Potter. It was likely that the turbaned man had gotten more than he bargained for with the experiment.

When he arrived in their residence, Olivia was pacing the living room. "Dad—

"Olivia," Severus held up a hand. "I need you to listen to me." He suddenly wasn't sure what do with his hands, or what to say. He settled for clasping his fingers behind his back.

Olivia waited a beat, and when he said nothing, her forehead wrinkled. "Our blood matched exactly, Dad. That would only happen if we were closely related. Like the same family related."

Severus ran a hand over his face. "You have the same biological parents," he said quietly.

Olivia had known it, somehow, before he said it. The moment she'd seen the potion. Not that there were many other options. Not like she could have been his mother or something.

"So if Harry is your son—

"Olivia." Her name came out so softly that if she hadn't been so heightened, every nerve exposed, she might not have heard it. Exhaustion dripped from every syllable. "He's not."

Olivia felt her world spin. "But we have the same father—

"You have the same biological parents," Severus repeated. He'd dreaded this conversation since the day she was born, and now that it was here, it was so much worse than he had imagined. "But I like to believe that I am your father, since I've raised you all this time."

Olivia sat hard on the sofa, her eyes burning. She wasn't totally sure which of the tumult of emotions she was feeling she should act on. She was so angry at the man who stood in the middle of the room. But yet she was so confused and worried and a little bit scared that she desperately wanted him to hold her and tell her that everything was going to be fine.

"Why?" Olivia whispered, taking a pillow from the end of the sofa and hugging it to her chest.

Severus approached carefully. In his nightmares, she had stormed out by now. Or dissolved into a puddle. She was showing her Slytherin childhood at that moment, and Snape couldn't decide if it was better or worse than all of the dramatic reactions he'd anticipated. He sat on the cushion beside her, trying to give her space but wanting to be close to his child. "This situation is so complex, Olivia. You'll have to be more specific."

"Why didn't you tell me?"

Severus pressed his lips together. "Tell you what? That the only father you've ever known isn't biologically related to you?"

"All of it. My mother is dead. I'm adopted. I have a brother."

"You knew your mother was dead," Severus countered.

"Convenient for you, wasn't it?" Olivia felt the anger bubble up.

Severus scowled. "Not particularly, no. I loved your mother very much."

"Which one?" Olivia held the pillow tighter and scooted closer to the armrest.

Severus sighed. "When Lily was pregnant, the Dark Lord was well on his way to taking over the wizarding world. We had reason to believe that he would target the Potter family, and most specifically Harry."

"Why him?"

"I can't tell you that." Severus said evenly. "The danger that surrounds him may very well not be gone. We were correct, obviously, that he was a target. He may still be. But Lily and James made what I'm sure was an incredibly difficult decision to protect you. Keeping you as separate from your brother as possible." He looked at his hands. "When Lily gave birth, I magically adopted you. In every way that matters, you became my child." He took a deep breath and looked at the green-eyed girl beside him. "You are my child. I lived a very private life at the time. We chose a pure-blooded woman who had recently passed to serve as a biological mother to make the story plausible." Severus wished desperately that Olivia would look at him. "There are very, very few people still alive that know that Lily delivered twins. And the ones that knew and are dead did not pass after living to a nice old age."

Olivia had tucked her knees up to her chest and was hugging them and the pillow. She stared at the toes of her shoes. The gravity of the discussion, if not obvious due to its content, could be assumed so by the fact that she had had her shoes on the furniture for nearly 10 minutes, and Severus hadn't mentioned it, or given her a pointed look, or swatted her feet off the cushion. "When you've told me about my mother…" She trailed off, not certain she wanted to know how deep Severus's deception had gone. Or if it even mattered.

"The only lie I've ever told you about her was her name," Severus said quietly. "And I suppose I'm guilty of allowing you to assume certain things, like our marriage. Or that she was a Slytherin. But Lily and I did grow up together. And I loved her very much. I love her very much, since we are being honest. Anything I've ever told you about her is true. I made sure of it." He paused and moved to sit just beyond her toes. "What has never, ever, for any moment, been a lie, is you are my daughter and I love you. And that will never change. I know it's a shock, and I know you're upset. I'm sorry for the circumstances, and I'm sorry that I had to keep it from you, but I'm not the least bit sorry that I have had the honor to be your father."

Olivia's tears spilled over, anger and sadness, and once they started, tears just kept rolling, down her face and soaking the pillow. "I hate you."

"I know," Severus said calmly. That was a phrase that he'd often prepared himself for when thinking of this moment. "I love you." It broke his heart that she was crying. It hurt more than anything she could say to him. He reached out to hold her and she flinched away.

"You should have told me," she accused.

"No," Severus disagreed, his voice flat, dethatched. "You couldn't know. You can't know." It took everything he had to raise his wand, but he knew he had to do it. It was too dangerous for her, too much of a burden. There was a flash of realization that crossed her face as she saw the gesture, but Snape was too fast.

"Obliviate."