Snape's old girlfriend comes back to Hogwarts because she was forced to drop out due to a vampire attack that turned her into a vampire. Malfoy's got a crush on Snape's vampire girlfriend and will do anything to win her heart.
Alice walked down a winding road in Diagon Alley, in her cat form. She was an animagus with the incredible gift to transform into any animal she chose. Alice knew she was fortunate to have this gift, but it came at a price. She was a vampire, or as the locals would've called her: a bloodsucking demon, leech and many other cruel names. She didn't care. She was sixteen, had been sixteen for eighteen years now and would remain sixteen forever until perhaps an angry villager would shout "Avada Kedavra," the only spell that could kill her.
She suddenly stopped walking. A boy, who had to have been around sixteen or seventeen had caught her eye. He had bleached blond hair and grey eyes, and was being dragged along by his worried looking mother. He had some followers though. The kids of about the same age were following him under an invisibility cloak. She couldn't see them, but she could smell them. Two boys and one girl, the girl was . . . muggleborn, and one of the boys was half-blood.
She suddenly became curious about why they were following this perculiar-looking boy. She decided to follow them. She wouldn't be able to keep up as a cat, so she hid in an alley and morphed into a bat. She flew overhead, just close enough that she could make out what was going on, but just far enough that they couldn't see her.
The three kids hid atop a building, trying to peer inside the one the boy had just entered. Alice, fortunate enough to currently be a bat, flew down and landed on the windowsill.
Inside there was a sort of ritual. She saw one of those dark mark tattoos on the boy's arm. So, he was a death eater. Voldemort had tried recruiting her. I suppose having a vampire on his side would help. But she'd turned him down. She wasn't on anyone's side, not his, nor the ministry's. Although she was good friends with Bellatrix Lestrange. And she'd had a relationship with Severus a while back. She was fourteen and he was nearly seventeen. They'd split up because of the age difference, he'd said. But yet somehow she was still looking foreward to seeing him when she returned to Hogwarts this year. She hadn't been there since she was human. Only Dumbledore knew she was a vampire and he was to tell the rest of the staff and students that she was a new student. But surely Severus would recognize her?
As the door opened and the blond boy stepped out, she flew off the windowsill up by the other three kids. She flew around their heads a few times. That was enough to scare the red-head boy, the other two following after him to calm him down.