Voldemort would be calculating. He might be using the Armistice as a way to regain his strength or plot a new strategy. His Death Eaters would be divided. Some might be happy to have a break from the fighting, while others would be eager to continue the war. Dumbledore's followers would be watching closely too. They would be the ones to keep the balance and make sure that the Armistice doesn't turn into a way for Voldemort to take over completely. The Ministry of Magic would be in a difficult position, trying to enforce the Armistice while also dealing with the different factions within the wizarding world.
The younger students at Hogwarts would be both curious and scared. They have heard stories of Voldemort's atrocities. They would look up to Harry Potter to keep them safe during this strange time of Armistice. The teachers at Hogwarts would be preparing for any eventuality. Some might be teaching the students how to defend against dark magic even during the Armistice, while others might be trying to promote peace and understanding between the different sides. The centaurs and other magical creatures in the Forbidden Forest would also be affected. They might be more or less likely to interact with the wizarding world depending on how they perceive the Armistice and whether it brings true peace or just a temporary lull in the fighting.
The other students at Hogwarts would be very afraid. Some might avoid Harry out of fear that Voldemort could use him to harm them. But Neville, being brave, would probably try to reach out to Harry in some way, believing that there's still good in him. The Order of the Phoenix would be in chaos at first. But then they'd start formulating plans. Lupin would be worried but also think rationally about how to deal with the situation, using his experience as a werewolf and a wizard.
In such a fanfic, one possibility could be that the Armistice is brokered by an unexpected third party. Maybe a powerful magical creature that both Harry Potter and Voldemort respect. This creature could convince Voldemort that the continuous war is only weakening the wizarding world as a whole and that there could be a better way to achieve his goals without all the bloodshed. Harry, on the other hand, would be skeptical but might see it as an opportunity to finally end the war and start rebuilding the wizarding community.
The teachers at Hogwarts would have diverse reactions. McGonagall might be disappointed but would still hope that Harry could resist the dark influence. Snape would be extremely complex in his reaction. Given his history with both Harry and Voldemort, he might see it as a sign of the prophecy coming full circle. The Death Eaters, on the other hand, would see Harry as a potential new leader. They might try to persuade him to embrace the dark side. As for Harry's other friends like Neville, he would be worried but would also stand by Harry, believing that he could make the right choice. The reactions of these characters would create a lot of drama and tension in the fanfic.
Ron would be extremely angry at first. He might feel betrayed by Harry. But as he realizes that something must be seriously wrong with Harry, he would probably join Hermione in trying to save Harry from Voldemort's influence. They would look for a way to break whatever spell or false belief has hold of Harry.
Hermione would be extremely shocked and disappointed. She has always been a firm believer in the fight against Voldemort and would see Harry's alliance as a huge betrayal. She would probably try to talk Harry out of it at first and if that fails, she might start her own investigation to find out what made Harry do this.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
Well, Ginny would be heartbroken that Harry left. She might have expected to build a life with him after Voldemort was defeated. But she's also a strong character, so she would likely throw herself into her own life, perhaps becoming a star Quidditch player. Neville would be overjoyed that Voldemort was dead. He might take on a role at Hogwarts, teaching herbology perhaps, and sharing his knowledge with the students. As for the other characters in the wizarding world, those who had suffered under Voldemort would be in a state of jubilation and start to pick up the pieces of their lives.
He'd probably be furious. Voldemort has a huge ego and most fanfiction would likely not portray him in the way he sees himself. He might view it as a form of disrespect to his 'true' nature and power.
Well, Harry Potter is a rather humble character. He might initially feel a bit embarrassed to see himself as the center of so many fictional stories created by fans. If the fanfic portrays him as a hero in new adventures, he would probably be honored. However, if it misrepresents his friends or the values he holds dear, he could be a bit indignant. He would likely approach it with an open mind but also with a sense of protectiveness towards his own story and those he loves.
The wizarding world would be in shock. Harry is seen as their last hope, so his surrender would make many lose faith. The Order of the Phoenix might go into a state of chaos, trying to figure out how to rescue Harry and continue the fight against Voldemort without him.