Maybe it's a power struggle. The OC could want more power and think betraying Zeref is the way to get it.
Perhaps the male OC was promised something better by another force or was manipulated into betraying Zeref. Maybe he had a change of heart about Zeref's methods or saw an opportunity to advance his own interests at Zeref's expense.
Yes, there are some really good Zeref stories in Fairy Tail. Zeref's backstory is quite fascinating. He was cursed with immortality and his actions throughout the series, like his relationship with Natsu, add a lot of depth to the overall plot.
There are fanfictions where Zeref interacts with other characters in unique ways. Maybe he forms an unlikely alliance with a Fairy Tail member. In these stories, the author can play around with the personalities and powers of different characters, creating exciting and unexpected plotlines.
One good Zeref story is about his origin. How he became cursed with immortality and how that changed his life. Another is his relationship with the demons he created. It shows his power and his inner struggle with his dark nature.
Well, it could be chaos. Zeref's dark powers would likely disrupt the harmony of Fairy Tail at first. His presence might cause a lot of suspicion and fear among the guild members.
In fanfiction, the idea of Mavis and Zeref having a child often leads to very complex and interesting storylines. Some fanfictions might explore the child's powers, which could be a combination of Mavis's magic and Zeref's unique abilities. For example, the child could have an extraordinary control over life and death magic, or possess some form of enhanced intelligence due to their parents' natures.
Well, it depends on the specific fanfiction. Some fanfictions do make Zeref and Natsu brothers. This can create some really cool plot twists. For instance, it could change how Natsu views his powers if he discovers he's related to Zeref. And it might also make Zeref more protective or possessive of Natsu in a different way than in the original story. It's a concept that allows for a lot of creativity in the fanfiction world.
The victory could be attributed to the development of the main characters' skills and their unwavering determination. Also, they might discover some hidden secrets about Zeref that give them an advantage.
One popular fanfiction could be 'The Sister's Redemption'. It might explore the relationship between Zeref and his sister in a unique way, perhaps showing how she tries to save him from his dark path. Another could be 'Sister of the Dark Mage', which may focus on her own powers and how she uses them in the context of the Fairy Tail world. And there's also 'Zeref's Sister: A Hidden Power', where her powers are a mystery that gradually unfolds throughout the story.
One possibility is that the fanfiction focuses on the aftermath of the wedding. How their union changes the world of Fairy Tail. Maybe it brings a new era of peace, as Zeref's power, combined with Mavis' wisdom, could have a positive impact on the magical balance. It could also show their life together, how they support each other, and perhaps start a family of their own, with their children having unique magical abilities.
One possible plot twist could be that Zeref has an ulterior motive for joining. Maybe he wants to use the guild's resources to further his own dark plans.