Voldemort could be a likely main antagonist. He's the most powerful dark wizard in the Harry Potter series. He might see the Avengers as a threat to his plans of total domination, not understanding their world but still seeing their power. So, he could use his dark magic to torture them, trying to break their will before eliminating them.
Another possibility could be a new character created specifically for this fanfiction. This character could be a powerful wizard who has a vendetta against both the magical world of Harry Potter and the superhero world of the Avengers. He could use his knowledge of both worlds' secrets to torture the Avengers, believing that they are responsible for some imbalance in the multiverse or something similar.
Perhaps in this fanfiction, the Death Eaters team up with some of the Avengers' villains. The Death Eaters could use the Cruciatus Curse on the Avengers, which is a form of torture in the Harry Potter world. This would be a really dark and intense scenario, with the Avengers having to figure out how to fight against this new and very different kind of threat.
It could be a powerful dark wizard who has somehow made his way into the Avengers' universe. Maybe a wizard who was banished from the wizarding world in Harry's universe and has now joined forces with some of the Avengers' villains.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
Yes, there might be some extremely dark and out - of - canon fanfictions out there with such a disturbing premise, but it goes against the general nature of the original story where Harry's parents loved him deeply. However, in the vast world of fanfiction, people can create all kinds of strange and often inappropriate scenarios.
I'm not entirely sure what specific situation you're referring to. In the Harry Potter series, Harry is the one who wears glasses, and he has his own share of hardships but not in a context of 'tortured and puts glasses on' in the original. In fanfiction, this could be some really out - of - the - box or dark interpretation where perhaps an author has created a scenario where Harry is tortured and then his glasses are put on him in a cruel or strange way. But without more context, it's hard to say exactly.
Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.
Another possibility is a corrupted member of the Ministry of Magic. They might be trying to use Harry as a scapegoat to gain more power within the Ministry. They could be working with the Death Eaters or acting on their own, but they would be a significant obstacle for Harry as he tries to stay alive and free while on the run.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.
It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.
It could be a new dark wizard who has risen to power. This wizard might see Harry as a threat to his plans, so he manipulates the situation to get Harry exiled.