It could be Orochimaru. He's always been interested in Naruto's power. He might use some new jutsu or experiment on Naruto to make him go berserk so he can study his power in that state.
A unique feature is the blend of the two different art styles. Naruto has a more traditional shonen art style, while Berserk is known for its detailed and often darker, more mature art. In the fanfic, this can translate to a different visual experience in the reader's mind. For example, Naruto's spiky hair might be combined with the more detailed and menacing look of Berserk characters.
In a 'naruto goes berserk fanfic', Naruto might lose control of his chakra in a new and extreme way. Maybe he's pushed to the edge by a powerful enemy that he can't defeat with his normal strength.
One common theme is the clash of values. Naruto comes from a world where friendship and perseverance are highly valued, while Berserk has a much darker and more cynical view of the world. So, in these fanfics, you often see Naruto trying to uphold his values in the face of Berserk's harsh reality.
One popular 'naruto berserk fanfic' is 'Naruto: The Berserker's Path'. It combines the elements of Naruto's world with the dark and intense atmosphere of Berserk. The story often focuses on Naruto developing some of the more extreme and brutal fighting styles inspired by Berserk, which makes for an interesting read.
One idea could be that Guts from Berserk is somehow transported to the Naruto universe. He could encounter Naruto and his friends. Maybe they initially mistake him for an enemy because of his intimidating appearance and weaponry. But as they get to know him, they realize his noble heart and his fight against the forces of evil in his own world, and they team up to face a common threat in the Naruto world.
It depends on the specific fanfic. Some fanfics might have Guts killing Griffith as a major plot point, while others could take a different direction and not have that happen at all.
One popular 'naruto berserk crossover fanfic' might be 'The Ninja of Midland'. In this fic, Naruto is somehow transported to the Berserk universe. He has to adapt to the dark and gritty world while still holding onto his ninja values. It's a really interesting read as it shows how Naruto's positive attitude clashes with the often hopeless and cruel world of Berserk.
The unique feature is the blend of two distinct universes. Naruto's universe is full of ninjas, chakra, and clans, while Berserk has a more medieval - ish, dark fantasy setting with demons and dark magic. In fanfiction, we can see how Naruto's abilities like shadow clones might be used in Berserk's battles against demons.
The power systems are quite different too. Naruto has chakra - based powers and unique jutsu, while Berserk has a more traditional medieval combat style with some elements of magic. Writers need to figure out how to balance these power systems when they bring the two universes together in a fanfiction. Maybe a Naruto character's jutsu could be adapted to fit the more brutal world of Berserk, or vice versa, but it's not an easy task.