The Darkness?

I think it was just the family that was supposed to die. The "children" or...the darkness resides in the house... I would assume.

Staying in the light is the only way they could survive. They all went in the dark and died.

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ummmm I thought the movie was stupid too but how can you say in a supernatural sense that a living consciousness to darkness is ridiculous when supernatural belief like religious belief is based on going out on a whim based on subjects that by their very nature is not natural?....

I agree. It seems it was rather the creatures who were hiding in the darkness rather than the darkness itself who attacked people (at the end, they were able to come out in the light, but still needed the dark to do whatever it was they were doing to the victims). And they did more than just kill them since there was no trace after the victims after they was taken into the dark. And the children who once were like all other normal children were now evil after they had been consumed by the force. Was their bodies ever found, or were they too taken by the dark? How was those in the old picture turned into monsters? Did the same thing happened to the mother, son, daughter and boyfriend?

This is not usual darkness, which we know as absence of light. This is another, so called "darkness".
I suppose that was essence, which they called "darkness", but really it was kind of infernal satanic artifical intelligence, but of mystical origin, which could take different forms, shapes and manage humans and read their thoughts and wishes. Like SkyNet of Terminator but mystical and magical. And this essence could use its powers only in spaces with absence of light. Maybe Satan/Devil was Creator and Administrator of this essence, so called "Darkness".And this is satanic project for occupation of Earth.The project was very succesfull and evil forces conquered Earth.