Typically, in Overlord vampire fanfiction, there are often elements of the vampire's immortality and thirst for blood combined with the power - hungry and hierarchical world of Overlord. The vampires usually have enhanced physical abilities like super strength and speed.
One possible main character could be a vampire hunter. This hunter might initially be after the forgotten overlord but then realizes that there are greater threats in the vampire world and teams up with the overlord. Another character could be a witch who has knowledge about the spell that made the overlord forgotten and helps him to break it.
One common theme is power. Since Ainz is already extremely powerful as an Overlord, making him a vampire in fanfiction often amplifies that power. He might gain new abilities like enhanced strength, speed, and the power to control blood. Another theme could be the struggle with his new vampiric nature. For example, he may have to deal with the urge to drink blood while still maintaining his sense of self. Also, relationships can be a theme. His interactions with his followers might change as they react to his new form, and there could be new dynamics with other characters in the Overlord universe who either fear or try to use his vampiric powers.
There's also 'The Vampire Lord Momonga'. This fanfic focuses on Momonga's transformation into a vampire and how it affects his relationships with the other denizens of Nazarick. It has some really cool battle scenes where his vampiric powers are on full display.
The plot could involve the vampire overlord being awakened from a long slumber. Maybe he discovers a world that has changed a great deal since he was last in power. He might have to deal with new enemies or form alliances with unexpected characters to regain his former status.
There is a fanfiction called 'Overlord: The Vampire's Dominion'. In this story, the vampire character has unique powers and a complex backstory that ties into the existing lore of Overlord. It delves deep into the political and power - hungry nature of the Overlord world from the perspective of a vampire. There are also many battles and strategic elements in it that make it quite engaging.
The combination of the powerful Momonga and the allure of vampirism. Vampires are often associated with power, immortality, and a certain mystery. When you add Momonga's existing character traits from Overlord, it creates a really interesting blend. His strategic mind as an overlord combined with the new abilities and weaknesses of a vampire can lead to some very exciting storylines.
His character might develop a sense of newfound superiority. As a vampire, he has additional powers, and this could lead to an inflated ego. For example, he may start to look down on his enemies even more. His moral compass could also shift. The need to feed on blood might make him more ruthless in his pursuit of power. He could also become more secretive, hiding his vampiric nature from those he doesn't trust, which changes his relationships with his allies.
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The Primal Chaos Tyrant Emperor was a fantasy novel, which included 'Tyrant Emperor',' Tyrant Martial Art','Primal Chaos Tyrant Heaven Art', and so on. The story told of the protagonist, Li Qiye, who had experienced various changes and challenges during his cultivation journey. He planted a green bamboo, raised a carp, and adopted a little girl. Then, he woke up to find that Cui Zhu had cultivated into a god, the carp had transformed into a golden dragon, and the little girl had become an existence of the nine worlds. This novel was full of immortal kings, useless mentors, bullying, sacrifice, and other elements, giving the readers an exciting storyline. However, the specific plot and ending could only be understood after reading the full text.