Complete Monster/Fable

The Fable series is known for its heinous main antagonists.

Fable

 * The first Fable had Jack of Blades as it's main antagonist. An ancient being from the Void who arrived in Albion millennia ago and wreaked multiple catastrophes upon humanity for refusing to bow to him and his cohorts, Jack survived his physical form's destruction by placing his soul in his mask and now body snatches whichever luckless man wears it. Jack razed the village of Oakvale, personally murdered the Hero of Oakvale's father, ripped out his little sister's eyes, and imprisoned his mother for decades of torture. When Jack meets the Hero as an adult, he tries to force him to duel his best friend to the death, he convinced Lady Grey to murder her older sister to become Mayor of Bowerstone, and later, after finding out his right-hand man, Maze, was killed, Jack says he planned on killing him anyway for outliving his usefulness. Jack's ultimate goal is to reclaim the Sword of Aeons, and he's been pursuing the Hero's family because the weapon's godlike power can only be activated by sacrificing members of their bloodline. Jack and his forces attack Albion to claim the sword, ending with him murdering the Hero's mother before his eyes. Jack then promises that after the Hero dies, the world will burn. In The Lost Chapters, Jack again cheats death through his mask and tries to corrupt the Hero into murdering his allies before attempting to steal his body as well. A being of pure evil who takes smug delight in his crimes, the shadow of Jack's evil would set the standard for Fables villains to follow.

Fable II

 * Fable II had Lord Lucien, who shoots and kills your sister in the beginning while you were children, before shooting you out the tower's window. He then works the kingdom to death to build a magical spire that would grant him his dark wish to purify the land. While driven mad by his loss of his wife and daughter, he has no respect of the life of others, increasingly so as the game wears on. While he is a Well-Intentioned Extremist in the beginning, he starts clearly fitting the trope by the game's end through Motive Decay. He also will murder the hero's family if he or she has one, telling the hero about it cold fashion, even gleefully. And finally, he shoots your loyal dog.
 * Lucien's experiment the Commandant is a sadistic bastard. He introduces himself by stating he has broken 272 men beforehand, proud of this. He then makes sure Lucien's prisoners are starved and makes you kill your only friend there. It feels so satisfying to kill him after this, but this is ruined by the revelation that he is not the only one of his kind. Bonus monster points to Lucien for making this guy.
 * Reaver. In the second game he manages to double cross the main character at least twice, was revealed to be the person responsible for the destruction of Oakvale, though he actually regretted that action in the past (key emphasis on the past tense in that statement). In the third game he controls the Bowerstone industrial sector, which he forces children to work in factories and shoots workers who slow or grow tired, protest, or break some unannounced rule he has yet to think up. He also acts as an advisor to the king in which his suggestions will be something along the lines of deforestation and tearing down a school to make room for a whorehouse.

Fable III

 * The Crawler from Fable III is pure death and destruction. If you don't need more convincing, when you first encounter it with Walter, it mind rapes Walter and then tries doing the same to you. After it forces it is revealed that it . Then after it attacks Albion  and . Most Eldritch Abomination kind of creatures would operate with Blue and Orange Morality and be exempt from this as a result. However, the Crawler takes glee in this (or at least its perception of morality is not properly explored) and qualifies.