- The Echthroi, intangible beings whose goal is to "X" (completely remove from existence) everything and everybody. If need be, they can perfectly impersonate someone, the disguise only penetrable by those who can summon the deepest feelings of true love for the person.
- The disease killing Charles Wallace, mitochronditis. While mitochondritis itself doesn't exist, mitochondrial myopathies—a group of neuromuscular diseases caused by damage to the mitochondria—do exist. And yes, they can kill you. Charles Wallace's illness bears a resemblance to MELAS, or mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes. Sleep well.
Nightmare Fuel / A Wind in the Door