Eater of Light

The Eaters of Light, described as light-eating locusts, were beings that fed on light. They came to our universe, as it was full of light.

They were a predatory species that hunted in large swarms. According to the Twelfth Doctor, there were potentially "millions" of them ready to emerge from their portal.

Biology
They resembled a large, reptilian quadruped with a frilled neck and scales. They had long, blue fluorescent tentacles which they used to hunt and locate prey, which was then dragged into their mouth. They were powered by consuming humans and using the energy to absorb vast amounts of light which would sustain them. A non-lethal blow of the tentacles would eject a black slime onto the victim and weaken them. If they reached Earth, they would have the power to drain the Sun and then every other star in the universe. (TV: The Eaters of Light)

History
In the 2nd century, the Picts used a "Keeper of the Gate" to keep the creatures from entering Earth's dimension. When the Romans invaded, the current Keeper, Kar, let one of the creatures through to destroy the army in the hopes that the creature would become weak and easy to defeat. When the creature got more powerful as a result, Kar thought she had doomed the world. The Twelfth Doctor, along with his companions Bill Potts and Nardole, worked with the Picts and the remaining Romans to drive the creature back into the portal by poisoning the light. To prevent any others coming through, Kar, the Romans and some Picts with instruments remained in the creature's dimension to fight them off. (TV: The Eaters of Light)