Another potential main character could be a Death Eater who has turned against Voldemort's ideology. This character could be exploring the world of dark creatures in order to find a way to atone for their past sins by using their knowledge of these creatures to fight against the real source of evil in the wizarding world, which could be the 'Lord of Dark Creatures' in this fanfiction.
It could also be an existing character from the Harry Potter series like Hagrid. Given his love for all creatures, including some that are considered dangerous, he could be at the center of a story about the 'Lord of Dark Creatures'. He might be trying to protect these creatures from those who would use them for evil purposes or trying to rehabilitate them.
Hermione could be one of the characters in the harem. Her intelligence would be an asset to Harry, even as a Dark Lord. She might be torn between her loyalty to the original Harry she knew and her feelings for this new, darker version. Ginny could also be part of it. Given her connection to Harry in the original story, her presence in the harem could add an interesting dynamic, especially if she tries to change Harry back to his old self.
Most likely Harry Potter himself would be a main character since he is in the title as the 'king of dark creatures'.
The Acromantula. Harry could come across a young Acromantula that is different from the rest of its kind. Instead of being aggressive, it shows an interest in learning about the human world. Harry, being the kind - hearted person he is, decides to befriend it. He might even teach it some basic spells or communicate with it using Parseltongue.
Hermione could be a likely character. In the original series, she was always by Harry's side. In this fanfiction, her intelligence could be a great complement to Harry's 'dark godlike' powers. Ginny Weasley is also a possibility, given her connection to Harry in the official story. And Luna Lovegood might be included too, with her unique perspective adding an interesting flavor to the harem dynamic.
Severus Snape's memories or a ghostly apparition of him could be a main character. Since he had such a complex relationship with the dark lord, he could offer insights to the daughter. Also, Ginny Weasley could be significant. She's a strong witch in her own right and could be a role model for the dark lord's daughter. And then there are the professors at Hogwarts like McGonagall who would have to deal with the situation of having the dark lord's daughter at school.
Potential characters could range from the well - known witches at Hogwarts to those from other magical institutions. Fleur Delacour could be in the harem, given her beauty and magical abilities. And there could be characters from the wizarding world outside of Britain too. Maybe a Japanese wizard or witch who has unique magical techniques and is drawn to the Harem Lord's power and charisma.
It could be a dark wizard who wants to use the power of magical creatures for his own evil purposes. He might be trying to control them, or use their parts for dark magic spells. For example, he could be after the horns of unicorns to make a potion that gives him immortality.
Hermione could be involved. She might follow Harry through time to try and stop him from becoming the Dark Lord or to fix the timeline if he already has. Dumbledore is another character. In the past, he could sense something strange about Harry's presence and try to guide him away from the dark path. Young Tom Riddle would definitely be involved. He could be either an ally or an adversary to Harry, depending on the plot. Maybe he sees Harry as a kindred spirit or as a threat to his own rise to power.
A less obvious candidate could be a corrupted Dumbledore. Imagine if he was swayed by the allure of absolute power and became the dark king. It would be a huge twist as he is seen as a symbol of good in the original books.
It could be a dark wizard who also has some form of time - related power. Maybe he wants to use time to his advantage to take over the wizarding world. For instance, he could go back in time to eliminate the founders of Hogwarts so that he can rewrite the rules of magic for his own evil purposes.