An interpretation of the film and unanswered questions (spoilers)...
***Spoilers, obviously***
I've watched this movie four times (one theater version, and three unrated DVD) and this is my broad interpretation of the film, particularly the ending.
Basically the house is a conduit for "The Darkness". The Darkness isn't demons, the darkness is it's own entity and it can manifest itself however it likes. I can manifest itself as one of your family members, a friend, or it can take on the form of a demon, anything.
Before the ritual is completed, the Darkness is confined to the house and it's powers are limited. It cannot manifest itself as loved ones or friends of yours. Also, it has no power *outside* of the house. However, after the ritual is completed, it's powers and the *domain* of those powers are exponentially increased, they are in fact, released upon the entire world.
After Regina (Anna Paquin) cuts her father's throat and he dies — completing the circle — the Darkness now has *vastly* increased powers that have no boundaries anymore (they can go outside the house now). The mother, Maria (Lena Olin) sees the Darkness take on the form of Regina and Paul (her children), they trick her into turning out the light and she dies. Regina and Paul break out a window and are tricked into thinking they are getting into Carlos' car (Regina's boyfriend), but really they are getting into a car with the Darkness.
The Darkness now can take on any form *in the light*. Remember, in the house, in the light, the mother is tricked into turning out the light by the Darkness taking on the form of her children and the Darkness pretends to be the mother to the children upstairs. One of the results of the ritual is that now the Darkness can take on the form of anyone (or thing) in the light. So, Regina and Paul get into the car with the Darkness and it drives them into a dark tunnel where (presumably) it kills them.
Regina and Paul DO leave the house, you can see the broken glass when the real Carlos shows up to the house. This is the POINT! The Darkness now is *unleashed* into the world. This is not a movie about a haunted house, this is a movie about a great, evil entity called the Darkness. This Darkness has now been loosed from it's boundaries (the house) into the world by the ritual now being completed.
Here is what the ritual has done:
1. The Darkness now is unleashed, it is no longer confined to the house. It now is loosed upon the entire *world*.
2. The Darkness can now take on the form of anyone, or anything, *in the light of day* to trick anyone into doing what it wants, which we assume is to get someone to turn out the lights or enter some dark area where it can kill or consume them. Every shadow in the world, day or night, is now it's domain as well.
3. Night is present, or is coming everywhere on the earth. So, within a 24 hour period, the Darkness will have the power to kill (or consume) almost anyone, or trick them into the dark to do so.
Here are the unanswered questions:
1. *WHY* would anyone unleash the Darkness? Albert (Giancarlo Giannini - Regina's grandfather) never tells her *why* he participated in the ritual. What does he *gain* from committing such a heinous act of killing his son? Did the Darkness pervert the seven parents into killing their children or where they promised power, money or some glory?
2. Who are the creepy old women in the photo? What is their purpose, are they the physical manifestation of the Darkness?
Here is the WORST mistake the film made:
The film never showed (adequately) the *new* power and scope of the Darkness. It didn't show that it's powers now are unleashed upon the entire world. It should have showed that it wasn't just about this *family*. The family was simply involved because the father is the last child to be sacrificed. This is an "end of the world" picture. The Darkness has now been unleashed upon the entire world and everyone in it is now susceptible to it's power. Sooner or later everyone in the world will be killed or consumed by the Darkness. Had the filmmakers made even just THAT part very clear, the film would have been much more well received (IMHO).
"...nothing is left of me, each time I see her..." - Catullus