"The Eaters of Light" is the tenth episode of the tenth series of the British science fiction television series Doctor Who. It was written by Rona Munro and broadcast on 17 June 2017 on BBC One. Munro previously wrote Survival, the final serial of the original run of Doctor Who, making her the only writer to date to have worked on the classic and revived eras of the show. The episode received generally positive reviews from television critics.

The Doctor (Peter Capaldi) takes Bill (Pearl Mackie) and Nardole (Matt Lucas) to second-century Scotland to settle their different theories about what happened to the ninth legion of the Roman army that vanished. When they arrive, however, they find an alien menace from another dimension that may be the reason behind the vanishing, presenting a far greater threat than any army.

The Twelfth Doctor and Bill, disagreeing about the fate of the Ninth Legion of the Imperial Roman army, travel in the TARDIS along with Nardole to the 2nd century in Scotland to prove the other wrong. Bill goes her own way to find the Legion, while the Doctor and Nardole look for their dead bodies.

Bill encounters some of the Legion's soldiers hiding underground, the TARDIS' translation circuits helping her communicate with them. The soldiers are hiding from a "Light Eating Locust" that seems drawn to any light source, killing those in its path. Meanwhile, the Doctor and Nardole discover the boneless corpses of the remaining Legion. They later come across a Pict tribe guarding a cairn and waiting for Kar, their leader and the "Guardian of the Gate". The Doctor impatiently enters the cairn, passing into an interdimensional portal full of creatures feeding off a light source. He comes out seconds later, but finds that more than two days have actually passed. Kar explains that once a generation, a warrior of their tribe goes through the cairn to defeat an "Eater of Light", but with the invading Roman army, she allowed one to escape to fight them. The Doctor warns her that unless they can get the creature back into the portal and close it, more of its kind will escape and consume the sun and all the stars in the universe.

Bill leads the surviving legion away from the creature and end up near the cairn, reuniting with the Doctor and Nardole. The Romans and Picts all agree their squabble is childish given the larger threat. The Doctor works out a plan to lure the Eater back to the portal during daylight. Once the creature is trapped, the Doctor tells them someone needs to stay within the portal until the sun extinguishes to prevent the creature from escaping, but as human life spans are too short, he prepares to enter the portal himself, since his Time Lord physiology and regenerative abilities will protect him. Bill objects and knocks him down, while Kar and the remaining Ninth Legion instead sacrifice themselves as a group to stop the creatures, despite the Doctor's objections.

With the portal closed, the remaining Picts honour Kar's memory in monumental stones and by teaching the crows to say her name, which Nardole observes continues into the present day. Back in the TARDIS, Missy awaits their return, to the surprise of Bill and Nardole.

Nardole mentions the Mary Celeste to Kar's followers as an example of people mysteriously disappearing. His tale of the ship's fate contradicts the First Doctor story The Chase (1965), where Daleks board the ship and frighten the crew into the water.

The read-through for "The Eaters of Light" was on 12 October 2016, with the main shooting taking place from 2 November to 22 November 2016.