It broke into their home and set up residence in their minds.

When the . . . . thing first insinuated itself into the Lund family household, they were bemused. Vaguely human-shaped, its constantly-changing cravings seemed disturbing, at first, but time and pressure have a way of normalizing the extreme. Wasn’t it always part of their lives?

As the family make more and greater sacrifices in service to the beast, the thrall that binds them begins to break down. Choices must be made. Prices must be paid. And the Lunds must pit their wits against a creature determined to never let them go.

It's psychological warfare. Sanity is optional.