Celestia is often paired with King Sombra. Their power levels are somewhat similar, and they represent opposite sides of good and evil, so it makes for an interesting dynamic. Twilight Sparkle is also a common character to be paired with him. Given Twilight's knowledge of magic, she could either be in a battle of wits with Sombra or trying to understand his magic for some reason.
Well, a plot could be that King Sombra is resurrected by some dark magic, but this time he has a different goal. Instead of just ruling Equestria through fear, he wants to find a way to break the cycle of hatred. He starts to interact with the ponies in a strange way, and they are both scared and intrigued. There could be a pony who tries to understand him and change his view of the world.
A common theme is the battle between good and evil. Twilight represents good with her magic and kind heart, while King Sombra is the embodiment of evil. Another theme is the power struggle. Twilight's magic power is often pitted against King Sombra's dark powers. For example, in many fanfictions, they have magical duels to determine the fate of Equestria.
Their relationship could start off as one of pure enmity. Twilight would see King Sombra as a great threat to Equestria. But as the story progresses, if they were forced to interact more, Twilight might start to see some small glimmers of humanity (or pony - ity) in Sombra. For example, she might notice that he has a love for a certain kind of magic that is not entirely evil.
In 'My Little Pony King Sombra x Fluttershy fanfiction', a prevalent theme is the exploration of good versus evil within the context of their relationship. Fluttershy represents good, and King Sombra represents evil. So, many stories will revolve around whether Fluttershy can influence King Sombra to turn towards the good side. Another theme is the clash of their worlds. King Sombra's dark and shadowy realm compared to Fluttershy's more idyllic and peaceful world. This contrast can lead to interesting plot developments as they interact and try to understand each other.
Yes, there are some out there. You can start by looking on fanfiction websites like Fimfiction. Just search for 'King Sombra x Fluttershy' and you'll likely find a variety of stories with different takes on their relationship.
Power is a common theme. Sombra is often depicted as a powerful and sometimes power - hungry character, so fanfictions explore how he wields or tries to regain his power. Another theme is redemption. Some fanfics try to give Sombra a chance at redemption, showing his possible change from a villain to a more positive figure. And there's also the theme of mystery as Sombra has a somewhat mysterious past that many fanfictions attempt to uncover.
The relationship could start with hostility. King Sombra being the dark entity he is, would see Twilight as a threat. But as Twilight tries to understand his motives, she could start to chip away at his cold exterior. For example, by showing him acts of kindness and mercy that he may not have experienced before.
One idea could be that Twilight discovers a way to redeem King Sombra. She uses her magic and knowledge to break through his dark exterior and find the good within him. Maybe she uncovers a tragic past that led him to become so evil.
One plot idea could be that Twilight has to enter the realm where King Sombra was banished to retrieve a powerful magic artifact. Along the way, she discovers secrets about Sombra's past that make her question if he was truly all evil. Another could be a time - travel plot where Twilight goes back in time and has to prevent King Sombra from ever rising to power in the first place. Maybe she teams up with an ancestor of one of her friends.