The Stonekeeper (Amulet #1) by Kazu Kibuishi

There's something strange behind the basement door...

After a family tragedy, Emily, Navin and their mother move to an ancestral home to start a new life. On the family's very first night in the mysterious house, Em and Navin's mom is kidnapped by a tentacled creature. Now it's up to Em and Navin to figure out how to set things right and save their mother's life!