The Stonekeeper: A Graphic Novel (Amulet #1)

After the tragic death of their father, Emily and Navin move with their mother to the home of her deceased great-grandfather. The strange house proves to be dangerous. Before long, a tentacled creature lures the kids' mom through a door in the basement. Em and Navin, desperate not to lose her, follow her into a fantastical world inhabited by robots, mysterious plants, and talking animals. It's up to Em and Navin to figure out how to set things right and save their mother's life!