The obvious key supporters would be the Death Eaters. They were always loyal to Voldemort. Bellatrix Lestrange would be one of the most prominent ones, being extremely fanatical in her devotion to him.
Well, the pure - blood families who believed in pure - blood supremacy were likely his key supporters. Families like the Malfoys, they had a long - standing belief in the superiority of pure - blood wizards. They would see Voldemort as a means to enforce their ideals on the wizarding world.
In the fanfiction where Voldemort won the first war, his key supporters were not only the Death Eaters but also some dark wizards who were attracted by his power. Some corrupt Ministry of Magic officials might also be among his supporters, seeing an opportunity to gain more power and influence under his regime.
If Voldemort won the first war in fanfiction, the social structure of the wizarding world would be turned upside down. Death Eaters would be in power everywhere. The use of Dark Magic would be widespread. The very fabric of the wizarding society that values love, friendship and equality would be torn apart. Wizards would live in constant fear, and any form of resistance would be brutally crushed.
There are quite a few. One popular one involves Voldemort using a powerful dark curse that the Order of the Phoenix couldn't counter.
One possible plotline could be a world under totalitarian rule. Voldemort would likely impose strict pure - blood supremacy laws. Muggle - borns would be persecuted and forced into hiding or slavery. Hogwarts might be turned into a place that only teaches the Dark Arts and Voldemort's ideology. Another plotline could involve the resistance. Some brave witches and wizards might form an underground movement to fight against Voldemort's regime, with their own struggles and sacrifices.
Bellatrix Lestrange would probably be a main character. As one of Voldemort's most loyal followers, she'd be at the forefront of his new regime. She might be in charge of torturing those who oppose Voldemort.
If Voldemort won the war in fanfiction, the wizarding world would experience a total transformation. First, the Ministry of Magic would be under his complete control. His Death Eaters would be in charge of all the important positions. The concept of justice would be warped, and any form of opposition would be brutally suppressed. The wizarding economy would likely suffer too as Voldemort's focus would be on power and domination rather than development. Also, the relationships between different magical creatures would be disrupted as Voldemort has no respect for any form of life other than his own pure - blood ideology.
Perhaps there's a fanfiction where Hermione disguises herself as one of Voldemort's supporters to infiltrate their ranks. She has to be extremely careful not to be discovered as she tries to find a way to stop a major plot they are hatching. Along the way, she discovers the complex reasons some people became supporters and uses that knowledge to her advantage.
It could lead to a completely different galactic order. The Sith's victory might mean a darker, more oppressive regime.
Maybe Voldemort manages to defeat all the major resistance. He could start by taking out key members of the Order of Phoenix one by one. For example, he might ambush Sirius Black early on, weakening the Order's intelligence network. Then, he'd target Dumbledore, perhaps using some dark magic that even Dumbledore couldn't counter. With the two biggest threats out of the way, he'd be able to overrun the wizarding world, starting with Hogwarts. He'd install his own regime, making pure - blood supremacy the law and punishing those who oppose him severely.
She might become more hardened and street - smart. Dealing with Voldemort's supporters would expose her to a darker side of the wizarding world. For example, she may have to make difficult moral choices, like whether to use unethical magic to save herself or others. This could make her question her own beliefs and values.
The plot could involve Tom being a heroic figure in the first war. Maybe he uses his unique skills or knowledge to outwit the enemy. For example, he could be a master strategist, coming up with plans that turn the tide of the war in his side's favor. He might also have some sort of special power or weapon that gives him an edge in battle.