Dawn is still sworn to secrecy during the Angelus Situation. She knows it's serious, because Buffy takes her aside and explains the basics to her in hushed tones, telling her to be very, very careful and to never, ever give Angel a new invitation to their home. She also tries very hard to impress upon Dawn the importance of staying indoors without actually saying "my ex-boyfriend is probably going to try to kill you". For all her worrying, it doesn't occur to her that Angel might do anything other than kill her.
She leaves her window open one night, feeling safe enough now that Willow's done the disinvite spell. In theory, an open window is no more dangerous than a closed one, since a vampire can't get through either. She supposes there are also run of the mill demons out and about, plus just plain immoral people, but it's hard to get too worked up about them when there's a superhero in the next room.
When she wakes up in the middle of the night, Angel is crouched outside the window. Dawn inhales sharply, but does nothing else- she knows she can't make a lot of noise, or Mom will wake up. Angel's wearing that blank look that he seems to wear most of the time, at least when he isn't evil. "Dawn, good, you're awake. Listen, I need you to invite me in. Buffy's in danger."
"I know," says Dawn, shifting away from the window. "From you."
"What are you talking about?" His puzzled expression is so normal, like he really doesn't know.
Dawn shakes her head. "Buffy had Willow do the disinvite spell. You're not welcome here."
"Dawn, I need to see her."
Dawn rolls over on her bed and fishes a wooden cross out of the drawer in her nightstand. Cross in hand, she plods over to the window. Angel's not even phased by the cross, and when she reaches up to close the window, he holds a hand up next to hers. "Are you sure you want to do that? You accidentally stick even one finger outside the house, and I'll pull you out and leave your corpse on the lawn for your sister to find in the morning."
Dawn freezes with her hand inches from his, nothing but an invisible barrier and the words come in keeping him from killing her. She pictures her dead body on the lawn, throat ripped out, the consequence of saying them or going outside the house. She pulls her hand back and reminds herself that it doesn't matter whether the window is open or closed. He can't get in.
Once Angel resigns himself to the fact that he isn't getting an invitation, he starts telling stories. He tells her about Drusilla, how he broke her before turning her. He tells her about Spike, and how Angel always outdid him in their contests of brutality. He tells her about the nuns he killed, and the things he did to them beforehand. He tells her about the children he's murdered, many of them younger than her. He tells her about all the various kinds of torture he's a master of, and how they affect the human body. In every story, she hears what he doesn't say- this is what I will do to your mother, this is what I will do to your sister, this is what I will do to you, and all because Buffy was kind to me, all because she made me feel like something other than a monster. She will pay for her mistake with her life, and with the lives of her family and friends.
Dawn watches him all night, cross in hand. She says nothing to him, because there's no snappy comeback for the kinds of things he's saying. He leaves just before sunrise. As soon as the first rays of sunlight stream into the room, Dawn walks over to her window and slams it shut.
She eats her breakfast in silence, at least until Mom comes in and forces her to give a few monosyllabic replies. Mom tousles her hair like she's six. "Someone woke up on the grumpy side this morning."
Dawn doesn't dignify that with a reply. When Buffy comes downstairs, Dawn silently passes her the box of cereal upon request, then goes back to staring at her own bowl of soggy goop.
"Honey, are you feeling all right?"
It's right on the tip of her tongue. No, my sister's ex-boyfriend is an evil vampire who wants to torture and kill us all, and I'm not sure I'm ever going to sleep again. I know Buffy's going to say he doesn't have a soul and it's not really Angel, it's just some demon wearing his face, but I don't care. He hates us. He hates us because we were nice to him, because we made him feel like the man he wasn't. Lots of vampires probably know where the slayer lives, but none of the others do this. Not even Spike. Angel lost his soul, and the human part of him got scooped out, and now nothing will stop him from torturing all of us to death.
"I'm fine."
She doesn't even tell Buffy about the incident. It isn't until years later she realizes she wasn't the actual target; the whole point was to throw Buffy off balance by making her fear for her sister. Since Dawn never told anyone, Angel wasted pretty much his whole night on something that didn't end up giving him a tactical advantage. In that sense, Dawn was the winner- not that it counts for anything, since the whole thing is a false memory in the first place.
She has nightmares about it anyway.