It could be Dumbledore from beyond the grave. He might find a way to reach out and summon Harry, believing that only Harry has the potential to truly defeat Voldemort in this new, dire situation.
In some fanfics, it could be the magic itself that summons Harry. The very essence of magic in the wizarding world, sensing the imbalance and destruction caused by Voldemort, calls upon Harry, as he has always been a symbol of hope and resistance against the Dark Lord.
Hermione could be a main character. She might be trying to find a way to rescue Harry from the timeout situation. Ron could also be there, supporting Hermione in her efforts.
I have no immediate knowledge of that particular fanfic. Maybe try asking in dedicated fanfic communities or using advanced search filters on fanfic platforms to increase your chances of finding it.
It could be Bellatrix Lestrange. She was always a fanatic follower of Voldemort and hated those who she thought were against the Dark Lord. Since Snape was a double - agent, she might have discovered his true loyalties and decided to kill him in the fanfic.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
In this fanfic, perhaps there could be a powerful magical artifact that summons Harry from a different time or dimension just for the purpose of facing Voldemort. The summoned Harry might have enhanced powers or knowledge. He could use new spells and strategies that he didn't have in the original story to take on Voldemort.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.
Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
Well, the plot might involve Voldemort's brother being raised in a different part of the wizarding world, completely unaware of his relation to Voldemort. But then, through a series of events, he finds out. He might then struggle with the knowledge that his brother is the most feared dark wizard. He could try to redeem Voldemort at first, but when that fails, he devotes himself to helping Harry Potter and his friends defeat Voldemort. He might have some unique magical abilities that prove crucial in the final battle.
It could be Jon Snow. He had to grow up a lot in the series and leave behind his more boyish ways. In a fanfic with this title, it would make sense for him to be the main character as he constantly had to make tough decisions that forced him to shed his innocence.
Perhaps a new dark wizard or witch who sees Harry as a threat. Since Harry has the power of Voldemort version 2, this new antagonist might fear that Harry will stop their plans for world domination. They could be someone who was in the shadows during Voldemort's time and now wants to take advantage of the chaos that Harry's transformation brings. They would try to use Harry's new identity against him and turn the wizarding world against him even more.