Amulet (9 book series) by Kazu Kibuishi

After the tragic death of their father, Emily and Navin move with their mother to the home of her deceased great-grandfather, but the strange house proves to be dangerous. Before long, a sinister creature lures the kids’ mom through a door in the basement. Em and Navin, desperate not to lose her, follow her into an underground world inhabited by demons, robots, and talking animals.