The Ancient Evil is the first thing Leo meets upon waking up inside the cabin. The Ancient Evil is made entirely of black goo and can change his form
The ancient evil is completely black except for his two glowing red eyes and smile. He can take any form, but typically takes a somewhat humanoid form with 5 long black tentacles for legs. He can fit inside any crevice and can grow infinitely large.
The Ancient Evil is immortal and can shape shift into any form. He can move objects telepathically as well as teleport. Though he can grow or shrink to any size, no matter what form he is in he will always be mostly black, though there can be small pieces of red mixed in.
The Ancient Evil was created along with the universe, and is responsible for every apocalyptic event that has ever occurred, such as the end of the dinosaurs or the Black Plague. He is not actually "evil". He simply tries to protect the balance of the universe. However one family deeply invested in the dark arts tried to kill him anyways. The Ancient Evil killed everyone in the family but two, one being a small boy named Ktimothy, the other being Ktimothy's father, who he banished to Limbo instead. However the method he used to kill the family, an adaptation of the Black Plague, began to spread and he failed to stop it, causing the outbreak of Leo's Plague.