Midnight entity

The Midnight entity was the term given to an unidentified lifeform of unknown origin encountered on the planet Midnight. It was one of the few beings that the Doctor had no knowledge of at all, which, given his expansive, encyclopaedic knowledge of the universe and everything in it across time, was a testament either to its obscurity, or its extraneousness. The creature was never given a name, nor did it reveal its true form.

Biography
During the Crusader Tours to the Sapphire Waterfall, a tour bus broke down. While looking out a window, one of the drivers noticed a shadow moving far in the distance. It then ripped the cockpit from the bus, killing the drivers.

The entity knocked on the shuttle and then, when it had found a host, somehow entered the shuttle without any sign of a hole in the vehicle. It chose to possess Sky Silvestry. The entity seemed to gather the strength of its host and then repeated every word a person said in exact precisions, including the square root of Pi to 30 decimal places. Eventually, the entity copied word-for-word at the exact same time, focusing on the Doctor, and eventually stealing his voice. She convinced the rest of the crew to try to throw the Doctor off the bus, convincing them that he was now possessed. However, the Hostess threw herself and Sky off the shuttle to save everyone when she noticed that the entity was using the Doctor's words.

Despite this, the Doctor had no idea if this killed the entity or if there were more. (DW: Midnight)

Powers and Abilities

 * The full extent of the entity's power is unknown. Indeed, it is unknown exactly how many of the events in the episode the entity was directly responsible for. It is also unknown whether the entity is part of a species, or existed alone on the planet.

The Midnight entity was capable of surviving on the planet Midnight, despite the fact that the Xtonic rays should have killed any being on the surface. It was capable of shaking a Crusader 50 bus, causing dents in its walls and ripping off the cockpit.

The Midnight entity could also possess a Human. It would start to copy what the people around it said, then speak at the same time and then predict what people would say. The Doctor believed it was trying to learn about its surroundings. The being seemed to be able to learn at an incredibly rapid pace and absorbed the passengers’ speech without forgetting, hesitating, or making a single mistake. After singling out the Doctor as a target, the entity was able to paralyse him and force him to repeat the entity's speech.

The entity within Sky also said, more to itself than the others, that that's how it "does it - makes you fight", indicating that the xenophobia and paranoia was at least partially caused by the creature, for unclear reasons. The rest of the passengers seem to immediately relax once Sky and the Hostess had been dragged outside, though this might have been merely of the shock of seeing the two of them die. (DW: Midnight)

Behind the scenes

 * This is the first time on Doctor Who where the true nature of an enemy has never been revealed. The name of the creature is not revealed in the episode, and "Midnight entity" is just an unofficial name.
 * The entity seems almost identical to the species in the Doctor Who Unbound audio drama A Storm of Angels, right down to the gemstone planet it inhabited.