Complete Monster/Sleepy Hollow

In the franchise with demons at every corner, there are some that stand out as especially heinous

* Moloch, the Big Bad of Seasons 1 and 2, is a ruthless and relentless Fallen Angel whose goal is to unleash the Apocalypse as well as destroy and subjugate all who live. Trapped in Purgatory, he steadily corrupts humans to act as his servants, including Ichabod Crane's best friend Abraham Van Brunt, who becomes the Horseman of Death in return for Moloch promising him Ichabod's beloved Katrina, whom Moloch imprisons in Purgatory. Throughout the years, Moloch turns humans into monsters and orchestrates murder after murder, punishing failure with brutal execution or torture. He then steals Ichabod and Katrina's son Jeremy, turning him into the Horseman of War and setting him against his parents while informing Jeremy he is only a servant who should follow his will.
* The Pied Piper from Season 2's "Go Where I Send Thee" is an extremely sinister demon who is known for mass slaughter of soldiers and for what it does with children. Each generation, the Piper preys on a daughter of a specific family; the child, when she turns 10, is kidnapped and taken to the Piper's lair to await her fate, which is usually her bones being made into the Piper's instruments. What's worse, however, is what happens when the girl is rescued. Out of nothing but spite, the Piper proceeds to infect all the girl's siblings with a fatal illness to force the parents to either return the chosen girl to the Piper or watch their other children die. While the Piper was betrayed by his employer, his Revenge by Proxy far outweighs this excuse; considering he seemed to enjoy killing and made a Deal with the Devil beforehand, he was psychotic from the start. Even in Sleepy Hollow, the Piper remains one of the worst Ichabod and Abbie have ever faced.
* The Hidden One, from Season 3, is a cruel, arrogant, and hot-tempered deity driven by lust for power and a hatred of mankind. Ten thousand years ago, due to his evil nature, his fellow gods banished him to the Catacombs, where he seduced Pandora, promising he would make her his wife if she freed him. Giving her a box which could steal a god's power, he sent her to slaughter his fellow deities, leaving him as the one all-powerful god. However, using a mystical artifact to strip him of his power, the humans imprisoned him in the Catacombs. In the present, Pandora managed to free him; however, due to Abbie, his power source was lost. Obsessed with restoring himself, he had his minions summon monsters from all over world to Sleepy Hollow so he could feed off them, causing many deaths. When the Kindred and Kindress instead fell in love, the furious Hidden One murdered them. Despite initial appearances, the Hidden One proved to only care about power, mistreating and berating Pandora. Attempting to kill the Witnesses, the Hidden One trapped himself in the same spell that originally defeated him, where he learned that, acting in good faith, Pandora had told the original humans of its existence. Not caring that his wife saved him for a second time, the Hidden One proceeded to torture Pandora. When Jenny tried to stop him from restoring himself, he cursed her boyfriend Joe Corbin to become a Wendigo once again, forcing Jenny to kill Joe. Finally regaining his former powers, the Hidden One imprisoned Pandora, then bragged how he would wipe mankind out gradually so he could bask in their fear and suffering.
* Malcolm Dreyfuss, the Big Bad of Season 4, was an internet tech billionaire obsessed with gaining immortality. Originally a harmless, nerdish man who was forced into a Deal with the Devil, to avoid keeping his end of the deal, Dreyfuss sought the Philosopher's Stone; his pursuits caused numerous deaths, including that of Diana Thomas's partner and friend Eric. His efforts climaxed with him attempting to sacrifice over a hundred thousand people. Gaining his immortality, convinced he was destined for greatness, Dreyfuss sought to conquer America; to accomplish this, he raised The Four Horsemen of the Apocalypse; spreading a supernatural plague through Washington, causing multiple deaths through spontaneous combustion or having their soul devoured, for his Horseman of Pestilence, then releasing a Chinese hunger demon which killed several people through either starvation or cannibalism to raise his Horseman of Famine. His plans succeeding, he turned Diana into his Horseman of War, effectively killing her. Dreyfuss then attacked Camp David and used the Four Horsemen to slaughter the entire US Army, forcing the government to crown him immortal dictator of America. He proceeded to ruthlessly suppress all freedoms and execute all dissidents, using the Four Horseman as his personal hit squad. Unable to kill the Witnesses, Dreyfuss had Ichabod imprisoned and adopted Diana's daughter Molly, rechristening her "Laura"; he then spent years lying and manipulating Laura until she learned the truth during an assault to wipe out the last of the resistance against Dreyfuss's regime. Realizing that Laura was going to attempt to undo his handiwork, Dreyfuss immediately tried to murder her. Despite his claims, excuses, and seemingly earnest façade, Dreyfuss was a callous, ruthless and fundamentally selfish man who proved that sometimes humans can be just as evil as demons.
* Jobe, a Prince of Pain from Hell's sixth circle, served as Dreyfuss's assistant, bodyguard and assassin throughout the final season. Implementing the majority of Dreyfuss's plans, he never missed an opportunity to commit murder, often choosing particularly painful methods for his victims. It was Jobe who originally shanghaied Dreyfuss into a deal for his soul, setting him off on the path to becoming a monster. Without Dreyfuss's knowledge, Jobe manipulated the terms of the deal to also send his best friend, the completely innocent Ansel, to Hell. He personally joined his fellow demons in torturing Ansel for twenty years, even carving his name into his chest. Assisting Dreyfuss every step of the way, Jobe murdered Mr. Wong in such a brutal manner it was mistaken for an animal attack; tortured Jake Wells for information, partially through physical pain, partially by giving Jake a nightmarish vision of him (Jobe) murdering Alex; and raised an army of Hessians as Draugurs to restore the Headless Horseman. Having no more use for them, he sent them to burn Sleepy Hollow to the ground. Disliking the prospect of serving Dreyfuss for eternity, Jobe helped bring about his downfall and happily dragged the screaming Dreyfuss into Hell. Beneath his professionalism and tranquil appearance, Jobe was an arrogant, disloyal, and sadistic monster.