Darklord
The Darklords are the rulers and prisoners of the plane of Ravenloft, from the D&D setting of the same name.
You might be wondering "How are they rulers and prisoners at the same time?" The reason for this is simple: each Darklord was pulled into the plane by the nebulous forces known only as the Dark Powers after an especially heinous deed, which reshape a part of the plane into a pocket realm tailored to their crimes. In his own realm, a Darklord has absolute power over everything except whatever they desire most- which is always ever so slightly out of reach to amplify their torment further.
Notable Darklords
Strahd von Zarovich
Darklord of Barovia, and the first Darklord to be introduced to the setting- in fact, its named after his own castle.
Originally the conqueror of a region of the same name, Strahd came to lust after a woman called Tatyana who rejected him in favor of his younger brother Sergei. Strahd took this very badly; badly enough that at some point he made what he called "a pact with Death" that turned him to a vampire in a desperate attempt to restore his youth. This too failed to win her over, and on the day Tatyana was to be married to Sergei, Strahd killed him and drove her to suicide.
His realm is Barovia itself (which was dragged into the mists with him), which he has absolute power over to the point where he can enter any private home uninvited (since he owns them all). However, he's cursed to have the events leading to his damnation repeat themselves forever- once every generation he will encounter a woman who he believes to be Tatyana's reincarnation, only to be rejected by her and become responsible for her death once more.
Azalin Rex
Darklord of Darkon. A powerful mage who inherited the rule of the city-state of Knurl, his draconic rule culminated in the execution of his son when he was caught freeing political prisoners. Soon afterward, Azalin became a lich and devoted himself to searching for a spell that could resurrect the dead, in the hopes that he could revive his son and retrain him to be as ruthless as Azalin himself.
As soon as he was taken into Ravenloft he lost the ability to learn any new magic at all- a terrible thing for any magic user, and even worse for a lich like him who became undead for the sole purpose of gaining more knowledge. This power over memory affects anyone else who enters Darkon as well; staying there for more than three months at a time causes a visitor's memories to be erased and replaced with false ones of spending one's whole life in Darkon. Originally an ally of Strahd when he first entered the mists, their relationship soured quickly after a failed attempt at escaping the Demi-plane of Dread temporarily split the former into two separate beings and they've remained enemies ever since. Another ill-fated attempt at breaking free in which he planned to become a demilich backfired even more spectacularly. Instead of allowing his essence to be freed from Ravenloft, it dispersed his essence across Darkon and destroyed the domain's capital. It took five years for him to reassume a corporeal form and take control of his realm again.
"Death"
Darklord of Necropolis (formerly Il-Aluk, the capital of Darkon). Originally a clone of Azalin made by him as part of a convoluted scheme to bypass his curse, he was transformed into a negative energy elemental by the prototype version of the "Doomsday Device" Azalin thought would make him into a demilich. When it backfired, the massive surge of negative energy it released killed the whole of Il-Aluk's population. "Death" then claimed the ruined capital and its remaining undead inhabitants as its own, and after Azalin reconstituted itself Necropolis became its own domain. A shroud of negative energy still remains over Necropolis, which instantly kills all non-undead which attempt to enter its borders.
Vlad Drakov
Darklord of Falkovia. He was originally the leader of the mercenaries called the Talons of the Hawk in Krynn. For their wanton brutality and bloodlust, they were taken into Ravenloft and claimed Falkovia for themselves (after a failed invasion of Darkon that was repulsed by Azalin's undead).
This rivalry with Azalin has since become Drakov's curse, though he seems unaware of it. While four other countries surround him, Drakov considers them "women and fops" not worth the effort of invasion and instead obsesses over an enemy he has no way of defeating. And even if he did somehow win, no Darklord can ever leave his own realm so he would never actually get to conquer it himself.
Dr. Victor Mordenheim/Adam
Darklord of Lamordia, of a sort. Like Frankenstein, Mordenheim sought to create life and was certain that a soul was not necessary for it to exist. To show him the error of his ways, the gods saw fit to endow the doctor's creation with a soul- specifically, an irredeemably evil soul. The newly created dread flesh golem was dubbed Adam, and it soon grew jealous of the love that his maker's wife and adopted daughter had for him and resolved to kidnap the former. This failed, and instead Adam killed the doctor's daughter and wounded his wife so badly she went into a vegetative state. It was at that time that the doctor and his creation were taken together into Ravenloft.
The doctor himself resides in his own mansion, obsessively trying to revive his comatose-but-alive wife (who has herself long since gone mad due to her life support system causing her constant pain simply by being active). Note that this is not out of love, though he still believes that it is- it has become nothing more than a compulsion that has become his only reason for living. Meanwhile, Adam is now the "ruler" of an inhospitable island aptly dubbed the Isle of Agony in the middle of Lamordia inhabited solely by himself, forever cut off from the acceptance that he sought, cursed to feel the doctor's pain as his own, and rejected by the very land that he supposedly controls. His only form of solace comes from sabotaging the doctor's attempts at reviving his wife just to spite him.
Dr. Frantisek Markov
Darklord of Markovia. Originally a pig farmer from Barovia, he grew increasingly obsessed with the anatomy of the pigs he butchered and began to perform bizarre experiments on them that wouldn't be out of place in a Haemonculus's laboratory. When his "subjects" inevitably died, he simply sold the meat without telling anyone what he had done to it first. Eventually his wife discovered his gruesome hobby and threatened to expose him, at which time she ended up becoming one of his experiments as well. After three days of vivisection, she finally expired- her corpse was so horrifically mangled that when it was first discovered it was mistaken for the remains of a monster. When Markov's involvement in her death became clear, the mists claimed him and made him a Darklord.
Fittingly, Markov's curse allows him to shapeshift into any animal form he wishes (save for his head, which remains human), but is incapable of assuming his original human form. As a result, he normally takes the shape of a gorilla in order to continue his increasingly deranged experiments without being impeded by a lack of thumbs. Said experiments culminated in the creation of the "Broken Ones"- animals (and the occasional unlucky human) that have been surgically mutilated into a human-animal hybrid form and given a semblance of intellect, similar to the Beast Folk of The Island of Dr. Moreau. Like said Beast Folk, they too are highly prone to reverting to their primal instincts and bestial appearance after only a few days.
Lord Wilfred Godefroy
Darklord of Mordent (the same place from the old Ravenloft II module). A nobleman who murdered his wife when she was unable to produce a son for him as an heir, and then murdered his daughter for trying to stop him. Their ghosts came back to haunt him for a year until he killed himself to make it stop, and when Azalin and Strahd inadvertently drew Mordent into the mists Godefroy (now a ghost himself) became its darklord. His curse is to be continually tormented by the ghosts of his wife and daughter just as he was in life, who tear him apart every night as they curse him for murdering him.
While he was formerly known to be rather passive as a Darklord, in recent times he has taken to enslaving other ghosts to do his bidding. In particular, he's had the mayor of Mordentshire under his thumb for years by holding the ghost of his wife hostage.
Hazlik
Darklord of Hazlan. A gay Red Wizard of Thay who was publicly humiliated and stripped of his status by his female rival and her boyfriend, the latter of which he lusted after. As revenge, he murdered the guy, made his girlfriend eat his corpse, then tortured her to death as well, at which time then was swept up by the mists. His curse is to suffer from nightmares of his now-inaccessible rivals defeating him with impunity and generally humiliating him every night, which has made him even more fixated on getting revenge against the Red Wizards. So he's crafting plans to cast a spell that will genocide all members of his race, the Mulan, throughout the multiverse. To escape being killed by said genocide spell himself, he's prepared to bodysnatch his female Rashemani apprentice when he's ready to use it.
Vecna
Yes, THAT Vecna.
The Maimed God has the dubious honor of being the only Darklord to force his way out of the Demi-Plane of Dread after being drawn in by the mists. The full story is recounted in the module Die Vecna Die!.
Gabrielle Aderre
Darklord of Invidia. A half-Vistani whose mother was raped by Drakov and was rejected by her fellow Vistani, she had always fantasized about her father's identity and resented her mother's unwillingness to speak of him, as well as her warning that if she bore a child it would end in disaster for everyone around her. When her mother was attacked by a werewolf, Gabrielle refused to aid her until she revealed the truth about her father. But when her mother did so, Gabrielle refused to believe it and left her to die. The mists took her after that, and she came to be cursed with an inability to cause direct harm to the Vistani, either physically or magically. Soon afterward she came to become the temporal leader of Invidia as well as its darklord, an opportunity that allowed her to begin persecuting the Vistani in spite of her curse.
She took many lovers as Invidia's ruler, though she only truly loved one of them, a wolfwere called Matton Blanchard. However, she was later seduced by the incubus calling itself "The Gentleman Caller" and left pregnant by him. Her son Malocchio soon proved to be one of the Dukkar- one of the rare males of Vistani blood with The Sight. On its own this would be a cause for concern among the Vistani, as the Dukkar is effectively the Vistani equivalent of the Antichrist; however, Gabrielle went even further and taught him to hate the Vistani as much as she did. Ultimately she taught him too well, as Malocchio betrayed Gabrielle and left her to die. She survived due to the intervention of Blanchard, but since then Malocchio has been the ruler of Invidia, persecuting the Vistani far more effectively than Gabrielle ever could have done. The only reason he has yet to kill her is because he knows that if she dies, he will inevitably inherit her title of darklord.