A great concept for a Black Butler demon OC fanfiction is a demon OC who has the power to manipulate time. This OC could have been a time - keeper in the demon realm before being exiled. They cross paths with Ciel and Sebastian, and their presence throws off the carefully planned events in the story. They might use their time - manipulating powers to try and change the fates of the characters, creating a complex web of new storylines.
One idea could be to create a demon OC who was once a human with a tragic past, like a wronged artist. The OC could have unique demonic powers related to art, such as being able to bring paintings to life in a twisted way. Another idea is a demon OC that is an outcast among demons because they have a sense of morality that is different from the norm in the demon world. They could befriend Ciel in an unexpected way and help him not just for the soul contract but out of genuine care.
You could create a demon OC that is a shapeshifter with a special connection to animals. Maybe this OC was once a hunter in their human life and has retained some of those skills as a demon. They can shift into different animals to gather information or to fight. In the fanfiction, they could be initially an enemy to Ciel and Sebastian, but as the story progresses, they form an alliance to face a greater threat in the underworld.
Chapter 15, "The Autistic Teen Jiang Shixu", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Shixu woke up from those nightmares filled with blood and...
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Chapter 231, "vLet Me Check Out the Strength of 5000", is the clearest early answer because it turns Huo's situation into a concrete plot event. It opens with Huo Xuan felt a burst of secret delight in her heart. and closes on A cool sports car was one hundred thousand bucks..
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Chapter 348, "Schemes and Counter-Schemes, the Five of East Ridge, Zhu Yang's Threat, The Dust Settles (Part 4)", is the clearest early answer because it turns East Ridge's situation into a concrete plot event. The chapter gives readers a real answer by opening on Upon hearing that Yanggu had sent someone first, Lu Yang raised his... and ending on At this point in the competition, the champion is actually already....
Chapter 278, "Thousand-eyed Demon Sheep, Terrifying Power, Spiritual Medicine 3", is the clearest early answer because the rescue effort creates visible on-page results. The setup is The biggest problem with this unexpected disaster was obviously that..., but the pressure keeps building through They picked a Cold Beast whose strength was far superior to theirs;....
Chapter 347, "Mirror Valley Council, Power Surge, Dajue Temple, Eight Battalion Tournament (Part 4)", is the clearest early answer because it turns Xia Chuan's situation into a concrete plot event. The chapter gives readers a real answer by opening on The six of them, upon hearing Xia Chuan's words, remembered the... and ending on If he had gone directly under the identity of Great Xia, Dajue Temple....
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Chapter 290, "Bountiful Harvest—Store Land or Store People", is the clearest early answer because it turns Xia Chuan's situation into a concrete plot event. The setup is Entering the Wushuang Realm, realizing that Long Mountain could no..., but the pressure keeps building through Finally, with an hour left until dawn, when the main force was only....