The Bone Door by Frances White

When Hop awakens in an ancient labyrinth, he has no memory of his life or how, indeed, he got here.
He does not recognise the mysterious girl trapped with him.
And he certainly cannot identify the shadowy figure stalking him, whispering terrible things . . .
But there is one thing he is certain of.
He must escape.
The only way out of the labyrinth is through The Bone Door. But it lies behind a series of locked doors hidden across an array of realms.
As Hop travels deeper into the maze, he discovers that he and his companions may be more connected to the place and its horrors than he could ever imagine.