There could be some disillusioned wizards who have lost faith in the current state of the wizarding world. They might see the Dark Prince as a way to change things, even if his methods are dark. These wizards could be those who have been marginalized or have suffered losses during the war. They believe that the Dark Prince has a vision for a different kind of wizarding society, not realizing the full extent of his dark intentions.
In this fanfiction, the Dark Prince might find allies among those who have been wronged by the Ministry of Magic. If there are wizards who have been unjustly punished or whose families have been persecuted, they could be swayed to join the Dark Prince. There could also be some international dark wizards who see an opportunity to expand their influence in the British wizarding world through an alliance with the Dark Prince. Another possibility is that there are some dark entities from other dimensions or realms that the Dark Prince manages to summon and convince to be his allies.
Draco Malfoy could be a candidate in some fanfiction. He was always on the edge of the dark side, being from a pure - blood family with Death Eater ties. In some fan - created stories, he might fully embrace the dark and become a sort of 'prince' figure in the dark kingdom.
One potential ally could be Neji. Neji has his Byakugan and his skills in combat. He could see the dark swordsman as an equal in terms of combat ability and form an alliance. Also, Tenten, who is fascinated with weapons. The dark swordsman's sword and his skills with it could draw Tenten's interest, and she might become an ally. They could exchange knowledge about different weapons and fighting techniques.
Hermione would definitely be one of his allies. She is smart and resourceful and would be able to help Harry navigate the new political and magical situations he faces as a prince. For example, she could research ancient laws and prophecies related to his new status.
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.
Voldemort could also play an important role. Since Harry is now the dark prince, there could be a complex relationship between them. Voldemort might see Harry as a potential ally or a threat, and their interactions would surely be a major aspect of the fanfiction, along with how Harry navigates this new relationship with the most feared dark wizard.
The Order of the Light could be a new enemy. They are a group of fanatical warriors who believe that the Dark Prince is an abomination and must be destroyed. They use powerful light - based magic that is harmful to the Dark Prince and his dark powers.
Spyro's original friends like Cynder could be potential allies. She has a connection with Spyro and might be able to understand his dark side and help him control it.
Snape himself could also be considered as the 'Prince of Slytherin' in some fanfictions. Given his complex relationship with both Slytherin house and Harry, a fanfiction could explore a scenario where he is given this title due to his deep connection to Slytherin's heritage and his own role as a double agent, which in a way makes him a sort of prince - like figure within the Slytherin world.
The Ministry of Magic would likely be an enemy. They are all about maintaining order and the Demon Prince's actions might be seen as a threat to that order.