Well, in the MLP lore, King Sombra's origin has to do with his dark magic. He emerged as a threat to the harmony in Equestria. He was drawn to the power of the Crystal Heart in the Crystal Empire. He managed to take over the empire, using his dark magic to oppress the ponies. However, his actions were not left unopposed. Celestia and Luna, the princesses, tried to stop him, which led to his first defeat and exile. But his story didn't end there as his influence still haunted the Crystal Empire.
There could be many fanfiction stories about King Sombra kidnapping Fluttershy. In some, Fluttershy's gentle nature might be a contrast to King Sombra's dark and powerful presence. Maybe the story could involve the other ponies in Equestria trying to rescue her. They might use their unique talents like Rainbow Dash's speed or Twilight Sparkle's magic to outwit King Sombra.
In terms of the plot, it could start with Sombra's return or some sort of threat that he poses. Luna then steps in to deal with it. The setting within the My Little Pony world would be important. It could be set in Canterlot or some other location. The fanfic might also introduce new characters or use existing ones to support the main plotline between Luna and Sombra.
Power and corruption is another common theme. King Sombra was power - hungry in the original story, and fanfics expand on this. They might show how his thirst for power not only affects himself but also those around him. It can also explore how power can corrupt even the best of intentions.
King Sombra started as a normal - ish unicorn in the Crystal Empire. But his thirst for power got the better of him. He began to study dark magic and became more and more powerful. He saw the Crystal Heart as a means to total control. So, he took over the empire, turning it into a desolate place. His origin is really about how power can corrupt a pony.
There are indeed some interesting ones. However, the quality can vary greatly. Some might focus on a more antagonistic relationship between King Sombra and Twilight, where Twilight has to use her magic and wits to overcome the threats posed by Sombra. Others could take a more unexpected turn and show a possible alliance or even a romantic relationship in a non - traditional sense.
One common theme could be the battle between light and dark. Luna represents the moon and has an association with the night, which can be seen as a time of mystery and sometimes danger. King Sombra is a dark character. So the fanfic may explore this contrast in terms of their powers and personalities.
I also heard about 'Sombra's New Reign'. This fanfic takes a different approach where instead of being completely evil, Sombra tries to rule in a more just way, which causes a whole new set of challenges and adventures for the ponies in Equestria. It's a great read if you like to see different interpretations of the character.
One common theme is power struggle. King Sombra is a powerful and often menacing character, and Twilight is also very powerful in her own right. So, their interactions can be centered around who has more power or how they can use their powers against each other.