Imagine sitting on a train when you suddenly see the lights flicker. At first, it doesn't bother you, but then you see a bandaged creature shuffling your way. You call out to other people around you, but no one can see what you are seeing. But, you can see it clearly -- a mummy creature is coming for you. What do you do? Whatever you decide, it's gotta be quick. You only have 66 seconds until you die.
The Foretold made its first and only appearance in the season 8 episode "Mummy on the Orient Express" and forced the Twelfth Doctor to work on a tight time schedule to discover what unseen force was dropping people like flies on a space train/laboratory. After using a few more passenger's deaths as case studies (much to Clara's dismay), Twelve eventually took on the time clock of a woman and discovered that the Foretold was a not a mummy, but a man of war. A victim of malfunctioning stealth technology, the Foretold endlessly continued his crusade by feeding on the mentally and emotionally weak. The Doctor was able to defeat him by "surrendering" to the Foretold, making him think the war is over. but a foe who singles out the weak, approaches them ominously while invisible to others, and gives them a heart attack is still pretty damn scary.
The Eleventh Doctor had the impossible task of trying to defeat this memory-wiping alien when he first encountered them during season 6 in the two-part story "The Impossible Astronaut"/"Day of the Moon." Known simply as the Silence, their looks alone are enough to make the toughest person shudder. With towering frames, giant heads, deep eye sockets, shriveled skin, and guttural voices (despite not having mouths most of the time), these otherworldly abominations are the last thing anyone wants to see shuffling towards them in the dark.
Of course, looks are only a small part of why they are so damn scary. The Silence have been running the Earth since the beginning of time, and were originally created by the Church of the Silence (Papal Mainframe) to allow people to confess their sins without remembering. Their memory wiping abilities allowed them to manipulate events without consequence, since everyone who saw them would forget everything as soon as they looked away. The only catch: As soon as someone turns to look at them again, they'll remember everything.
Outside of scaring the hell out of a person multiple times, the Silence could harness electrical current to zap a victim into dust! Yikes. Not even their keen fashion sense can forgive The Silence from their most horrible deed: helping Madame Kovarian kidnap a baby River Song to warp the child into a Doctor-killing weapon. Not cool, guys.
Shadows are kind of spooky on their own, but flesh-eating shadows that can strip an organism down to a skeleton in seconds are on another level. According to the Tenth Doctor, the shadow race known as the Vashta Nerada ("the shadows that melt the flesh") are everywhere, but remain harmless in small groups. But, when faced with a handful of people in an abandoned library, the Vashta Nerada became a carnivorous army who were protecting their forest (aka the paper in the books made from their tree homes) and devouring anything that dared step into the dark.
Ten, Donna and others were managed to stay out of the shadows for a time. Unforunately, they witnessed one crew member's haunting last thoughts, which were channeled through her spacesuit after she was attacked. Another poor sap started freaking out when he realized he had two shadows -- a clear sign of his impending death. The Vashta Nerada also led to the "death" of River Song, who at the time was still an enigma from the Doctor's future. If there's a "Save the Rainforest" analogy in here somewhere, it's lost in the terror of what are basically swarms of tiny invisible piranhas.