Doctor Who S10 E10 The Eaters of Light

"Space. For all."

A long time ago, the ninth legion of the Roman army vanished into the mists of Scotland. Bill has a theory about what happened, and the Doctor has a time machine. But when they arrive in ancient Aberdeenshire, what they find is a far greater threat than any army. In a cairn, on a hillside, is a doorway leading to the end of the world.