Thanks to buffyworld.com for their excellent transcripts of episodes, without which this story wouldn't have been possible.
Innocence Lost
She wandered aimlessly for what seemed like hours, dry-eyed but entirely hollowed out with grief. Finding herself back at the school after dark had fallen, she sat on the steps near the entrance. Lights beckoned from the interior where Giles and the others would be meeting to decide what to do about the Judge, but she wasn't ready to face them yet. Instead, she wrapped her arms tightly around herself and rocked miserably.
Everything had been so perfect last night. Angel had been a tender and gentle lover, taking care that the momentary pain, the lovely cruelty that ended her girlhood, hadn't surprised her. His cool hands had soothed her when she had burned for him. She thought that nothing could possibly be as wonderful as loving him had been.
And then today he had been so changed. So callous about their night together, passing it off as though it hadn't been one of the most important events of her life. First he'd laughed at her innocence and inexperience with sex, and then he'd practically called her a whore. She didn't know what she'd done wrong.
After chasing her thoughts around the same circle a dozen or more times, she decided she might as well be miserable inside. Maybe the others would now have some idea how they could deal with the Judge and she could think about that instead. She got up to go inside and all the lights in the school went out.
Suddenly afraid, she ran for the door. She arrived just in time to see Angel grab Willow around the throat from behind while Ms. Calendar stood at the other end of the hallway, brandishing a cross. At that moment, Xander also burst through the doors from the student lounge.
"Don't do that!" Xander shouted.
"Oh, I think I do that," Angel replied, and she could hear the same coldness in his voice she had heard that afternoon. She slipped quietly through the doors.
"Angel," Willow choked, as he tightened his grip.
"He's not Angel anymore," Ms. Calendar said. "Are you?"
"Wrong," he replied slyly. "I am Angel. At last." Willow struggled as his grip on her throat tightened.
"Oh my god," blurted Xander.
"I've got a message for Buffy," Angel said menacingly.
She stepped forward into the illumination cast by the school's emergency lights. "Why don't you give it to me yourself?" she asked, struggling to keep her voice steady.
Angel spun around to face her, but didn't loosen his grip. "Well, it's not really the kind of message you tell," he threatened. "It sort of involves finding the bodies of all your friends." Willow yelped with pain as his fingers tightened with these words.
This wasn't happening, she insisted to herself. He can't- "This can't be you," she said in anguish.
"Gee, we already covered that subject," he replied sarcastically.
She had to keep trying to reach him. "Angel, there must be some part of you inside that still remembers who you are," she pleaded. Beyond him she saw Xander take the cross from Ms. Calendar and begin sneaking up behind him.
"Dream on, schoolgirl," he said, mocking her. "Your boyfriend is dead. You're all going to join him."
She moved forward, trying desperately to come up with a plan to get him away from Willow. "Leave Willow alone, and deal with me."
"But she's so cute," he said, pinching her cheek hard like some deranged uncle come to visit. "And helpless. Really a turn on."
Behind him, Xander crept closer. She couldn't let him risk himself too, and still hoped she might reach Angel somehow. "Xander, no!" she cried as he was about to attack.
Angel - Angelus - spun around and clouted Xander in the side of the head with his free hand, knocking him to the floor where he lay dazed. "Thanks for the warning, sweetheart," he growled, then he lunged forward and his fangs ripped at Willow's throat. She didn't even have time to scream. Behind her, Ms. Calendar collapsed and shrieked as though Willow's lost scream had been forced out of her instead. Bright arterial blood spurted over them, staining everyone crimson. Willow crumpled to the floor in a spreading pool.
Angelus turned next to her and grabbed her by her shoulders, drawing her near. His face was wet and his breath reeked with fresh blood. "Things are about to get very interesting," he whispered roughly, then forced a kiss on her, smearing her with her friend's life. Shoving her away into the wall, he whirled and was gone.
Xander crawled across the gore-smeared floor to cradle Willow's lifeless body in his arms. "Oh Willow. Oh no," he repeated over and over, out of his mind with grief.
She felt herself sliding slowly down the wall, unable to speak, think, or even to breathe. Great hitching sobs shook her body. If she hadn't tried to stop Xander, if she had been here instead of wallowing in self-pity all afternoon . . . if she hadn't slept with him. Willow's death, and all the destruction Angelus would cause before she stopped him - it was and it would be all her fault.
Buffy woke to a tearing sensation of loss and misery. She hadn't felt this empty and desolate since her mom had died, and maybe not even then, since her mom's death hadn't been her fault no matter how horrible it had been. But in her nightmare, Willow had died directly because of actions she had taken, and she still felt cold with that knowledge.
Despite understanding consciously that it had been only a dream, she knew she wouldn't be able to sleep again until she checked in on Willow. Buffy crept silently down the hall and eased open the door of the master bedroom that Willow had until recently shared with Tara. She let slip a sigh of relief as she heard her friend's slow, regular breathing.
Though she had tried to be as quiet as possible, Willow still stirred from her sleep. "Buffy?" she asked groggily, "What's wrong?"
"Nothing, Will," she replied. "I just had a bad dream and wanted to see if you were okay. Go back to sleep and I'll talk to you in the morning."
"G'night," said Willow, mostly to her pillow, as Buffy closed the door softly behind her to return to her own room.
I hope this isn't going to become a regular event, Buffy sighed inwardly as she sat up in bed against her own pillows, waiting for sleep to return. A chilling thought took her. These nightmares weren't prophetic, exactly, since the events in them had already happened - or hadn't, quite. But the way that her memories were being twisted in the dreams was disturbing enough that she resolved to get Willow, Xander and Anya looking into possible mystical disturbances. If the dreams were warnings of something big to come, she wanted to be ready for it.
Having resolved to take action in the morning, Buffy was able to slip back into sleep's embrace at last.