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.
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 great mlp king sombra fanfic is 'The Shadow's Redemption'. It delves into the character of King Sombra, exploring his backstory and a possible path to redemption. The story has some really interesting plot twists and well - developed characters.
One great MLP Sombra fanfic could be 'The Redemption of Sombra'. In this fanfic, Sombra is given a chance to reform and find his place in Equestria. It shows his internal struggles and how the ponies around him gradually accept his change. It's really engaging as it delves deep into Sombra's character, which is often overlooked in the original series.
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.
Their relationship could develop in a more complex way. Maybe Sombra has a hidden past that Luna discovers, which makes her pity him. This could lead to her trying to help him reform. Sombra, on the other hand, might be initially distrustful but then gradually open up to Luna as she shows him kindness. They could end up as friends or at least allies who have a deep understanding of each other.