A plot could revolve around a love triangle involving the Undertaker, a Black Butler character, and an outsider. The Undertaker's feelings for the character are complicated by the presence of the outsider. The story could explore themes of jealousy, sacrifice, and the power of love as the characters navigate this complex relationship.
One idea could be that the Undertaker is secretly protecting the main character of Black Butler out of love. He uses his unique skills and knowledge to keep them safe from various threats in the dark underworld of the story.
One interesting plot could be Undertaker's past being explored in more detail. Maybe he had a relationship with one of the other main characters' ancestors that comes to light and affects the present story. For example, if he had a connection with Ciel's family long ago.
One idea could be that the reader is a new undertaker's apprentice. They start off being clumsy and Undertaker finds it both amusing and endearing. As time passes, a deep bond forms between them, and they face various supernatural challenges together in the world of Black Butler.
One plot idea could be that Vincent, before his death, made a secret pact with Undertaker that is only revealed years later. For example, it could be related to the future of the Phantomhive family and the underworld they are involved in. Another could be a story where Undertaker has memories of Vincent that he has never shared, and these memories start to surface when he meets Ciel, causing him to act in unexpected ways towards Ciel. Maybe there's a plot where Vincent's spirit is trapped in a limbo-like state, and only Undertaker can free him, but it comes at a great cost to the balance of the afterlife world in the Black Butler universe.
One interesting 'Black Butler Undertaker fanfiction' story could be about Undertaker's past and his connection to the Phantomhive family in more depth. Maybe it reveals secrets that were only hinted at in the original series.
In a world of "Black Butler", Undertaker once fell in love with a kind - hearted maiden. She was a healer who often visited the graveyards where Undertaker lurked. Their eyes met one day among the tombstones. He was drawn to her gentle nature and the way she treated the dead with respect. They started to exchange secret glances and small talk. But their love was doomed as she was from a different world, one that couldn't accept Undertaker's macabre ways. Eventually, she was forced to leave, leaving Undertaker with a broken heart and a longing that would never fade.
One plot idea could be Sebastian falling in love with a human who has the power to see demons' true forms. This creates a unique dynamic as he has to protect her from other demons while also dealing with his growing feelings. Another idea is a time - travel plot where Sebastian goes back in time and meets the love interest in a different era, perhaps in a more innocent time for him. And a third could be a story where the love interest is a powerful witch who challenges Sebastian's loyalty to Ciel in the context of their relationship.
One interesting plot could be Grell and Undertaker teaming up to solve a mystery in the underworld of the Black Butler universe. Maybe they discover a new kind of demon or a secret about the reapers that has been hidden for ages.
One interesting story might be about the Undertaker's mysterious past and how it intersects with the main plot of Black Butler. Maybe it delves into his connection to the reapers or his true motives for getting involved with the characters in the series.
The Undertaker's character can be developed by showing his vulnerability in love. For example, he might be hesitant to show his true feelings at first because of his dark past and the secrets he holds. But as the love story progresses, he could start to open up more and let his guard down.
One idea could be a new character coming into the Phantomhive household as a potential mate for Sebastian. This character could be a half - demon with unique powers and a mysterious past. Their presence would disrupt the usual dynamic between Sebastian and Ciel, creating all sorts of interesting conflicts and power struggles.