To make his arc believable, also include his relationships. His relationship with Palpatine as his Sith master could be complex. At first, he blindly follows Palpatine's orders, but as he grows in power, he might start to suspect Palpatine's true motives. This internal conflict, along with his changing view of the dark side, would create a rich and believable character arc for Sith Anakin.
As he spends more time as a Sith, he might start to feel a sense of emptiness. He realizes that the power he sought isn't fulfilling. He could then have a moment of self - reflection. Maybe he meets a character who challenges his beliefs, like a young Force - sensitive who still believes in the good in the galaxy. This makes him question his path and start to seek a way back from the dark side.
One idea could be Anakin as a Sith having to face his former Jedi allies. He might struggle with his new identity while still having remnants of his Jedi self. For example, he could be sent on a mission by the Sith to destroy a Jedi stronghold but find himself hesitating when he sees his old friends there.
Padmé is often paired with Anakin in fanfiction. Even though he has turned to the Sith, their love story still has a lot of potential for exploration. Writers might create scenarios where Padmé tries to bring Anakin back from the dark side. Another popular character is Obi - Wan. Their relationship as master and apprentice can be further developed in the context of Anakin's Sith journey. There could be intense confrontations or moments of reluctant understanding between them.
Perhaps a fanfiction where Anakin, as a Sith, is sent on a mission by Palpatine. But on this mission, he meets a young Force - sensitive who reminds him of his own past. This makes him start to have doubts about the Sith ways. He begins to secretly teach the young one about the Force in a more balanced way, all while trying to hide it from Palpatine.
First, think about their backstory. Maybe they were once a Jedi who was seduced by the dark side. For appearance, give them a menacing look, like dark robes and a red lightsaber. Their personality could be very ambitious and power - hungry.
A Sith Lord in your Star Wars OC fanfiction could have a lair on a desolate planet. It could be filled with ancient Sith artifacts. When it comes to their motives, it could be as simple as wanting to rule the galaxy or more complex like seeking revenge for something that happened in the past. You can also give them a distinct voice, like a deep, raspy tone that sends shivers down the spines of those who hear it.
One memorable OC Sith could be Darth Zyra. She has a very distinct look with her purple - hued lightsaber and her elaborate black and red robes. Her power lies in her ability to manipulate the minds of her enemies, which she uses to great effect during battles in the Clone Wars.
First, create a unique Sith character. Think about their backstory, like where they came from and what made them turn to the dark side. Then, plan the plot. It could involve the character's rise in the Sith hierarchy or a conflict with a Jedi. Also, don't forget to add details about their powers and how they use them. For example, if your OC has a special lightsaber technique, describe it vividly.
The SI could also be a love interest for Anakin. In a fanfiction, this could be a very different kind of relationship compared to what he had with Padmé. This relationship could either be a positive influence, giving Anakin more stability and a reason to stay on the light side, or it could be a source of conflict, especially if the SI has their own secrets or agendas.
Well, for a Sith Pureblood OC in a Star Wars fanfic, you could make her Zara. Zara has a very distinct look with her black horns and violet eyes. She was trained from a young age in the ways of the Sith, but she secretly yearns for a different path. Maybe she has an encounter with a Jedi who makes her question her Sith teachings. This could lead to an interesting internal struggle within her character throughout the fanfic.
She could be developed through her interactions with Sith apprentices. Maybe she tries to redeem one of them, showing her compassion and strong will. This would also show her growth from her Jedi training.