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Well let me start by saying it isn't a separate "series", but it was added after chapter 99, it's chapter 99.5 and the chapter is called Berserk Prototype. The characters look the same and the little girl looks like Griffith. Guts are better IMO, he's more...human I guess would be a good way to describe him. He seems more caring and more human, plus the eye-patch makes more sense than constantly having his missing eye always closed. This definitely looks, smells, and feels like berserk but it tastes a bit different. Worth 5-10 min to read.
There are a few fights in this chapter. Guts easily kills some bandits then later faces off against a giant demon--an apostle of Vana. In this story, Guts is seeking revenge against the apostles for having torn feir mother to pieces. The general purpose of the chapter--of showing how Guts is on a self-destructive quest for revenge, how fe is super strong, and how the world has monsters in it--is the same as the first chapter of the main series. The biggest difference seems to be that this oneshot implies that the apostles are those Guts is seeking revenge against, but (if I remember correctly) in the main series the ultimate object of feir revenge is the five members of the Godhand and fe fights the apostles mainly because they are followers of the Godhand. Guts' facial expressions in this oneshot look kinda dorky. Fe's animated and comes across as more emotional, like an angsty teenager or something. The fights look pretty cool.