I think it is talking about being with a narcissist.
"There's a crack in the mirror and a bloodstain on the bed"
Narcissists are often viewed as vampires because they suck the life force out of people, and they are constantly projecting an image of themselves, so she sees a crack in the image that they try to project almost like the true self is coming out.
"There's a rocking chair by the window down the hall"
It could mean looking outside of herself seeing what she let this person do to her, the lyric "I hear something in the shadow down the hall" could mean a shadow of the person she once was.
"They used to dance in the garden in the middle of the night"
That could mean all of the promises that were made.
I think it is talking about being with a narcissist.
"There's a crack in the mirror and a bloodstain on the bed"
Narcissists are often viewed as vampires because they suck the life force out of people, and they are constantly projecting an image of themselves, so she sees a crack in the image that they try to project almost like the true self is coming out.
"There's a rocking chair by the window down the hall"
It could mean looking outside of herself seeing what she let this person do to her, the lyric "I hear something in the shadow down the hall" could mean a shadow of the person she once was.
"They used to dance in the garden in the middle of the night"
That could mean all of the promises that were made.
Anyway that is what I think it means.