The TARDIS arrives in second-century Scotland, where the Doctor plans to solve the mystery of the vanished Ninth Roman Legion. The travellers find the soldiers massacred, their bodies in a state of decay akin to decades of sunlight deprivation. The Doctor and Nardole are captured by the Picts who were fighting the Roman incursion, while Bill is rescued from an alien predator by the survivors of the Lost Legion. An interdimensional portal, hidden within a stone cairn, holds the secret to defeating the monster -- but conflict between the Picts and the Romans threatens to doom the world.

Since writing Survival, the last story of Doctor Who's original 1963-1989 run, Rona Munro had become a celebrated playwright. As a result, although Steven Moffat had wanted to ask her for a new script since becoming Doctor Who's executive producer in 2008, he assumed that she would no longer be interested in contributing to the show. When he finally approached Munro, he was delighted to discover that this was not the case, and she agreed to develop a script for Season Thirty-Six. In particular, she asked to write an adventure with an historical setting, and Moffat suggested that she consider scenarios which would be appealing to a younger audience.

Drawing upon her own interests as a child growing up in Aberdeen, Scotland, Munro's thoughts turned to Rome's Ninth Spanish Legion. It was posted to Great Britain in the mid-first century, but had vanished from Roman records just a few decades later. Most scholars assumed that it had been annihilated in a conflict, although debate remained as to whether such a battle had occurred in Great Britain itself or following a redeployment on the continent. A popular belief was that the Legion had been decimated after an uprising by Scottish tribes; Munro recalled this theory from Rosemary Sutcliff's 1954 children's novel The Eagle Of The Ninth.

The Pictish Beast was an icon which appeared on numerous carved stones attributed to the Picts

Munro was also familiar with the Pictish Beast, a seahorse-like icon which appeared on numerous carved stones attributed to the Picts, who were the tribes of northern and eastern Scotland in the latter half of the first millennium. Their name probably came from the Latin for “painted”: a Roman allusion to the Picts' custom of elaborate tattooing. The Pictish lands were eventually subsumed into the Kingdom of Alba, and their culture into that of the Gaels. Given the remoteness of their territory, Munro imagined that the Pictish Beast might represent a real creature with which the Picts had come into contact.

The first draft of Munro's script, The Eaters Of Light, was completed in mid-May 2016. At this point, the adventure was intended to be the ninth episode of Season Thirty-Six. Kar was originally called Ke, while Nardole was not present in the initial version. The last warrior sent into the cairn, decades earlier, was seen to emerge from the portal, dead. One of the Picts, the elderly Tarr, explained that they had been lovers when she was a young woman. As the story evolved, Munro briefly had the Doctor engage in dialogue with the creature, who refused his entreaties to return through the portal. Late additions included the notion that Kar had deliberately freed the Eater of Light as revenge against the Romans, and the Doctor's intention to enter the portal to take over the responsibility for keeping the monsters at bay.

The Eaters Of Light was originally intended to be made as part of Season Thirty-Six's fifth production block. However, it was ultimately brought forward to form part of Block Four alongside episode five, Oxygen, under director Charles Palmer. The first day of recording for the adventure was scheduled ahead of the main shoot, on October 14th, when Bill's entry into the caves took place at Porth yr Ogor near Ystradfellte. Palmer then concentrated on Oxygen for several weeks.

His focus turned back to The Eaters Of Light on November 2nd, when Morlais Castle, a thirteenth-century ruin near Pant, was the site of the glen where the Doctor and Nardole found the dead Romans. On the 3rd and the 4th, the hillside entrance to the cairn could be found at Blaengarw Farm in Blaengarw. Following the weekend, filming resumed across the English border in Clearwell, Gloucestershire, with Bill encountering the Roman survivors at Clearwell Caves on November 7th and 8th. The moorland upon which the TARDIS materialised was actually the Cwm Cadlan quarry in Penderyn, where filming took place on the 9th. Events in the woods rounded off the week's work, with November 10th and 11th taking Palmer's team to Fforest Fawr near Taff's Well.

The remainder of The Eaters Of Light was almost entirely recorded at Roath Lock Studios in Cardiff, starting after the weekend on the set for the Picts' hall. These scenes were recorded from November 14th to 16th. Part of the latter day was then spent within the cairn, where Palmer's cameras continued to roll through to the 18th. The last material taped before the weekend was Bill's fall into the pit trap.

The last two days of principal photography on The Eaters Of Light were November 21st and 22nd. The first day began with action in the chamber below the hall, before Palmer's attention turned to the tunnel entrance. Work on the latter set continued on the second day, alongside pick-up shots of Nardole talking to the crow. More inserts were taped at Roath Lock on November 30th, after which establishing shots of the hall were captured near Clearwell Caves. Footage of the crows was recorded in the studio against a green screen on December 13th.

This should have marked the end of filming for The Eaters Of Light but, during January 2017, it was decided that the adventure would swap places in the broadcast schedule with the original episode ten, Empress Of Mars. With The Eaters Of Light now immediately preceding the two-part season finale, World Enough And Time / The Doctor Falls, Moffat wrote new concluding material involving Missy, the Doctor's Time Lord arch-nemesis, as part of the ongoing story arc involving her possible redemption. These sequences were filmed on the standing TARDIS set at Roath Lock on February 22nd and 23rd.

The Eaters Of Light was broadcast on June 17th. Doctor Who was brought forward by half an hour to 6.45pm to accommodate the debut of the singing competition Pitch Battle, which would now follow it for the rest of the season. Meanwhile, Pointless Celebrities returned as Doctor Who's lead-in; it had occupied the same place in the schedule earlier in the year. Unfortunately, the ratings stumbles that Doctor Who had been experiencing throughout Season Thirty-Six impacted The Eaters Of Light most of all. Its 4.7 million viewers represented the smallest Doctor Who audience since The Curse Of Fenric -- ironically, the serial which had preceded Survival back in 1989. However, the television landscape had changed significantly in the intervening twenty-eight years and, despite its rating, The Eaters Of Light was nonetheless the most-watched programme of the day.

• Doctor Who: The Complete History #88, 2019, “Story 274: The Eaters Of Light”, edited by Mark Wright, Hachette Partworks Ltd.