I'm not sure specifically as I haven't read that exact fanfic. But generally, if it involves 'Freedon Nadd' possessing an original character (oc), it might be about the changes in the oc's personality, abilities, or the story that unfolds due to this possession. Maybe it shows the oc being forced to do things against their will, or perhaps they find a way to fight back against the possession.
In a fanfic about Freedon Nadd in a reincarnation OC setting, the power dynamics would be crucial. Freedon Nadd was a powerful Sith Lord. If the OC is reincarnated in the same universe, they might have to deal with the lingering influence of Nadd. The OC could either be in a position to oppose him, perhaps with some unique power or knowledge from their past life, or they could be unknowingly influenced by him. This kind of interaction can lead to a great exploration of good vs evil and the struggle for power.
I'm sorry I can't really recommend similar fanfics off - hand as I don't have a database of fanfics related to Freedon Nadd possession. But you could try searching on fanfic - specific websites. Use keywords like 'possession', 'Freedon Nadd' and 'original character' to find ones that might be similar.
One way to incorporate Freedon Nadd is to have the OC discover an ancient artifact related to him. This could trigger memories or visions of Nadd's time, and set the OC on a journey to learn more about this powerful figure. For instance, the artifact could be a Sith amulet that once belonged to Nadd, and when the OC touches it, they start seeing glimpses of his past and the events he was involved in.
A really interesting 'freedon nadd sith oc fanfic' is 'Rise of the Nadd - Inspired Sith OC'. It details the origin story of an OC who discovers the teachings of Freedon Nadd. The fanfic is full of action - packed scenes as the OC battles against Jedi and other Sith rivals while trying to uphold the dark legacy of Nadd.
First, research Freedon Nadd's history and the Sith lore thoroughly. Then, create a unique original character with a distinct personality and backstory. For example, you could make your OC a former Jedi who was corrupted by Nadd's teachings.
Another plotline might involve Hermione being possessed by a time - traveling consciousness. This allows her to experience different eras of Hogwarts' history first - hand. She could then bring back important information from the past to help in the present, like how to defeat a powerful dark wizard that is starting to rise again in their time.
Often, when possessed by a dark force, she'll be shown as more menacing. Her eyes might change color, and she'll have an air of danger around her. Authors will make her act cruelly towards others, especially those she used to be friends with. This is a way to show the contrast between the normal Hermione and the possessed one.
Another theme might be the discovery of new powers or abilities that come with the possession. It could add an interesting twist to the normal Ben 10 power set. Also, there could be a theme of redemption if the possessor is initially a villain. The story could be about how the possession changes the relationship between Ben and his friends and family as they try to figure out what's wrong and how to fix it.
The unique feature is often the new powers Naruto gets. For example, in many 'possessed Naruto fanfic', he might gain dark or ancient powers that are not part of his normal abilities in the original series. This can lead to very different fight dynamics.