WARNING! THERE MAY BE UNMARKED SPOILERS!
The thing was able to inhabit both inanimate and animate objectsIf we accept it was a consciousness or an entity, able to inhabit things, the only thing it could possibly inhabit to suvive on the planet was diamonds, that's why it was so strong in the first place. It also purposefully seeks out the weak point, in both the outer casing and the people inside. It kills the mechanic and the driver simply to get to the door to see if it could get through and, when it couldn't, it was forced to transfer it's consciousness through the ship into Sky (hence the dent in the wall)
That thing was The First.Considering the fact that there would be trillions of humans by then it's reasonable that the evil in the universe would also increase. The existence of Evil keeps the First in existence so it could stand to reason that more evil would make it more powerful.
The Doctor was still contaminated... and may still be.OK, so the person who infected him is gone and he can talk again. Somehow, her being dead frees him, even thought he thing responsible for all this lived out where nothing can survive. We're talking about a guy who regularly repeats things. When his best friend, shortly after this, absorbs part of his mind, he suppresses all memory of him and all of his personality over her protests. We later find out that it isn't quite as dangerous as it seems if she remembers, though we never do see her get it all back. After her, he refuses to take any other companions and is constantly fearful, but also in some ways eager, for his "death." When he finally regenerated, he no longer seems concerned with any of this, eagerly seeking out a companion. Clearly he thinks he burned it out. But he's been wrong before.
The entity was a surviving creature of the Time War.A nameless horror that gleefully prods people into becoming monsters sounds like it would fit right in with "The Nightmare Child" or "The Skaro Degredations." But the real evidence for this is that when the Doctor restores the Timelock in "The End of Time Part 2," the music playing comes from the Series 4 track for this episode. The Emperor's ship fell through, so maybe the Midnight creature did as well.
The entity wasn't an entity, but a phenomenon.The Doctor said himself — they were the first to be in (and see optically) that specific region. The "shadow" was noticed by the mechanic, who (along with the pilot) was the first victim. The area enhances psychic fear, allowing it to manifest empathically and psychokinetically. The Doctor's uncertainty about the "shadow" caused the knocking; the increasing fear by the mechanic caused the cabin to get destroyed. The increasing fear about the knocking couldn't get "in" physically, but got in psychically. Skye's increasing panic made her the juiciest target; the Doctor's sudden jump in fear at being put under the microscope made him more vulnerable. Fear feeds on fear; fear experienced in that particular location of the planet amplifies it and gives it form, but it isn't sentient in any sense.
The entity was actually Skye the whole time, capable only of feeding on fear and causing fear-induced hallucinations.A plausible chain of events follow thusly.
Midnight is PSR J1719-1438 b.Both are planets made of diamond, whose star emits deadly radiation and should render the planet totally uninhabitable.
There was no entity...At least, not the one we think it is. The, for lack of a better phrase, "monster" was capable of really nothing, when you think about it. Disagree? Well, let's see what it did during the course of the episode:
The entity was a virus.Viruses can play dead until they find themselves a new host, then they resurrect and wreak havoc on the new host's body. This would enable the entity to survive in an environment that was previously assumed to be uninhabitable, because it would mean that technically it WAS uninhabited: the virus WAS dead, until that nice bus full of new hosts arrived to bring it back to life. In addition, viruses can affect the brain as well as the body, which would make it somewhat scarier because hysteria and hallucination could be symptoms of the virus. The only things I can think of against this WMG would be how the driver and the mechanic were killed in a way that involved the bus being ripped in half, and why the virus would affect humans and time lords similarly.
The Entity was a demon from before the current universe, similar to the Beast and Abaddon.Its body was imprisoned at the centre of the planet Midnight but, like the Beast, its mind escaped and was able to possess people, and had a large degree of telekinetic power, allowing it to knock and rip the cockpit off the truck. Like the Beast, it worked by sowing fear and, in this case, paranoia amongst its victims. The other powers it displayed, like voice stealing and being inherently disturbing, were just powers that it had where the other demons didn't, like how Abaddon killed with its shadow. Midnight's sheer inhospitability (no atmosphere and deadly sunlight) was a mechanism to keep people away from the demon and ensure that the act of physically escaping would kill it, like how Krop Tor was in orbit around a black hole and would fall in if the Beast escaped. Evidently, its references to the cold and the dark were references to its imprisonment.
Whatever the thing was, it was related to the Weeping Angels.They're both unfathomable and seemingly impossible, terrifying incarnate and never seen moving on-screen. Both of them have the Doctor scared, and manage to pull one over him rather easily(the Midnight creature mind-raped the Doctor and the Weeping Angels not only remain out there in their appearances, but in their most recent appearance? I need to cry to a pillow now). Like the Weeping Angels, the Midnight creature is a being of the abstract. While the Weeping Angels represent the idea of musical status, the Midnight creature is the living embodiment of the concept of repetition. Adding the above theory, The Beast himself may be a different abstract-namely, the embodiment of the idea of THE DEVIL, hence why he says he cannot die-he's not Satan as we understand him, but the very concept of Satan(which explains why he's got the Big Red Devil look going-he's just taking the apperance of what we expect for the Satan concept).
The Entity was the Great Intelligence and the Hostess was an echo of Clara.
The Midnight entity is the spirit of the BeastThe Beast boasted about it could never been destroyed. Sounded like showoff, but what if it's true? The Midnight entity was powerful, hated specialy the Doctor and took delight in the Mind Rape it put him trough.
The "Midnight" entity is Fenric.A bodiless Eldritch Abomination able to possess others and has a grudge against the Doctor? Seems obvious in hindsight.
The Midnight entity is...Irrelevant. The entity isn't the real monster of the story; humans are. That's why we never find out what, exactly, it is.
The Midnight entity is not evilWe know the entity absorbs, and copies what it sees. It took over Sky to do just that, it didn't mean it in an evil way. What did it observe as Sky? It saw people argue, fight, manipulate each other, back stab, plot to kill even. The midnight entity absorbed this, and believed that is how this race acted. Therefore, when Possessed!Sky started talking and telling them to kill The Doctor, it wasn't knowingly being evil. That's only what it observed.