Well, he might be really confused at first. Death Battle fanfiction often features extreme and out - of - character scenarios. Harry, being used to his own world's rules, would probably find some of the battles and character interpretations very strange.
Yes, it is quite popular. There are a number of fans who like to combine the two concepts. They love to see how the heroes and villains of Overwatch would fare in the intense battles characteristic of Death Battle. There are various online platforms where such fanfiction is shared and discussed, which shows that there is a significant audience for it.
Typically, it might feature the characters from strq observing the intense battles in Death Battle. There could be a lot of reaction elements, like shock, excitement or analysis from the strq characters.
Yes, it likely exists. There are fanfiction for all sorts of combinations in the world of fandoms. Since both Spiderman and Death Battle are popular topics, it's very possible that someone has written fanfiction that combines the two, and Spiderman could be depicted as reading or being involved with such fanfiction within that fictional story.
There are often fanfictions that involve crossovers in 'watches death battle' scenarios. Imagine a battle between a superhero from the DC universe and a powerful anime character. These crossovers can lead to really exciting and unexpected plot twists as the different power systems interact.
Maybe Spiderman gets some new combat ideas. He could see some unique fighting styles in the Death Battle fanfiction and incorporate them into his own superhero battles.
Obviously, Sonic himself is extremely popular. He's the main character, after all. Tails also often makes an appearance as Sonic's sidekick. Knuckles is another popular one, with his strength and connection to the Chaos Emeralds. And then there are the villains like Dr. Eggman who are frequently part of the death battle scenarios in these fanfictions.
He might be shocked. The fanfiction could show his friends or himself in situations that are completely different from what he knows. For instance, if it shows him losing to a minor villain, it would be very unexpected for him.
He might also be a bit confused at times. Some fanfictions deviate a lot from the real events in his life. For example, if there's a fanfiction where he has a completely different upbringing, it would be strange for him to read about. He could also feel a sense of flattery that so many people are interested in his story and want to create their own versions of it.
One popular one might be those that focus on unique character interactions during the 'death battle' scenarios in the Bleach universe. For example, a story where Ichigo has an unexpected alliance during a battle.