While Game of Thrones featured many heroic, valiant and courageous characters, it had just as many evil villains who wrought misery and destruction upon the continent of Westeros and beyond. These evil characters would be some of the most vicious and destructive Sith Lords if they ever found themselves in the Star Wars universe.
With the powers of the Dark Side of the Force at their disposal, there's no telling what kind of damage these individuals could do. Some would be fearsome warriors while others would be cunning manipulators and strategists who relied on dark powers to create chaos across countless worlds. Any one of them could represent a moral threat to the galaxy.
Viserys Targaryen
Viserys was an immature, ambitious and unhinged young noble incapable of thinking more than five minutes down the road. As such, he failed to comprehend how his actions would backfire and lead to his own death. Yet his evil and ambition would have fit perfectly with Sith ideology.
It's doubtful Viserys would last long in the Sith Order, however. It's highly probable that he'd serve as an apprentice to a powerful master who would in turn use him as a means to an end. When his usefulness runs out, Viserys would be betrayed in order to pave the way for a more competent apprentice.
Joffrey Baratheon
Some may see Joffrey Baratheon as an Emperor Palpatine-style character, but that's inaccurate. Despite the power to which he ascended, Joffrey was a petulant child who stewed in his own toxic cocktail of fear, resentment and self-doubt, which caused him to lash out in attempts to make himself look strong. The worst of Joffrey's behavior was directed toward Sansa Stark, who he enjoyed tormenting.
While these are certainly Dark Side traits, Joffrey was not mature enough to see the bigger picture, instead thinking only of amusing himself. As such, his fall to the Dark Side would be quick, but his lack of foresight and cunning would probably lead to his death at the hands of another Sith in relatively short order.
Cersei Lannister
Cersei Lannister could easily have fallen to the Dark Side of the Force, primarily due to her fear of losing her loved ones. This particular kind of fear caused many Jedi to lose their way, including, most famously, Anakin Skywalker. Without the ability to let go of everything she held dear, she left herself vulnerable.
In Game of Thrones, Cersei gets worse and worse as the story unfolds, primarily due to her falling into a darkness of her own making, but in the Star Wars universe, she'd be a prime candidate for the Sith. Her callous and cruel attitude toward friend and foe alike, along with her distorted concepts of love and compassion, would prove to be her undoing.
Melisandre
Like many who fall to the Dark Side of the Force in the Star Wars universe, Melisandre would believe that her actions were in pursuit of a greater good. Therefore, she'd think little of harnessing the powers of the Dark Side to achieve her desired outcome.
Melisandre was already gifted in the ways of magic, but with the Dark Side at her disposal, her corruption would have been complete. In service to her holy quest, she would have wrought unimaginable damage while never believing for an instant that she was the evil one.
Qyburn
Qyburn's lack of empathy and conscience was mixed with an innate curiosity to explore the dark arts, something for which the Sith Lords of old were well known. In fact, the Sith regularly dabbled in the arcane arts in an effort to create new species of warriors or to expand their powers.
Though Qyburn would not have been a Sith Lord who cared about ruling the galaxy, his introverted schemes and discoveries would have given the Sith Order a leg up on the competition. Personally, Qyburn would have done little damage, but over the course of time, his work could have caused immense destruction.
Tywin Lannister
Tywin Lannister was a pragmatic villain who did not believe he was evil, and he viewed his ruthless nature as necessary to achieve order. In many ways, he was the Game of Thrones equivalent of Count Dooku, who held similar views. They were also both born of nobility, one of many little-known facts about Count Dooku.
In Tywin's opinion, force and brutality are sometimes necessary, which is a common lie the Sith tell themselves to justify their actions. As such, Tywin would easily turn to the Dark Side, but his status as a Sith Lord would be built more on ruthless practicality as opposed to wickedness and evil.
Petyr Baelish
Petyr Baelish, aka Littlefinger, has many qualities that would lure him easily to the Dark Side, and he would be a powerful player in galactic events. His ability to scheme, manipulate, and sow seeds of doubt and discord led to some of Littlefinger's most notorious deeds and would prove invaluable to the Sith agenda.
In fact, Littlefinger would probably be able to hide his status as a Sith even from the most prying of eyes, and that would make him far more dangerous than a skilled lightsaber duelist. With the powers of the Dark Side at his disposal, he could cause catastrophic levels of damage.
Gregor Clegane
The infamous Mountain was undoubtedly one of the most despicable and evil characters in the entire Game of Thrones saga, and his fall to the Dark Side of the Force would be swift and efficient. With no remorse in his heart to speak of, Clegane was effectively a one-man army who valued bloodshed, conflict and destruction over anything else.
In many ways, Clegane would have become the Game of Thrones equivalent of Darth Maul, existing solely as an instrument to advance the will of his master. His huge size and hulking physicality was second only to his own evil nature, which would have been amplified through the embrace of the Dark Side.
Ramsay Bolton
If ever there was a case to be made for the Jedi striking first, Ramsay Bolton was it. This utterly unscrupulous sadist was devoid of a single redeeming quality, even going so far as to murder his own father to amass more power. With no connections to his family or loved ones, he would be a prime Sith candidate.
Bolton was also an aggressor who lived life according to his own lustful passions, which is a tenant of the Sith ideology. His level of power and influence in Westeros was surefire proof of his ambitious nature. If given the time and resources, Bolton could rise to the status of a Sith Lord with an apprentice serving under him.
The Night King
The Night King and his White Walkers are still shrouded in plenty of mystery, but one thing is for certain: He would be the most powerful Sith Lord of any Game of Thrones character, with no equal. His magical powers were already insurmountable on their own, and his combat prowess was frightening.
If this particular villain ever gained the powers of the Dark Side of the Force, he'd become a Sith Lord with enough terrible power to destroy entire planets, much like Star Wars Old Republic Sith Lords such as Darth Nihilus. The level of his destructive power could not be measured, which is a terrifying notion.