During the war between the Dragon Hunters and the Dragon Riders, Johann acquired enough Dragon Eye lenses to determine the location of the "King of Dragons". The Dragon Riders also were able to use Dragon Eye lenses to determine the King of Dragons' whereabouts - Berserker Island. The two factions clash at the island. Johann, Krogan, and Hiccup discover that not only is the "King of Dragons" a huge dragon living underneath the island but that she has an unhatched egg.
The Berserker Bewilderbeast has apparently been living beneath Berserker Island for some time, as the underground caverns were filled with her ice formations. Though she spends her time in a deep sleep, her presence nonetheless causes partial disruption by making any mature dragons unable or unwilling to fly into the centre of the island.
Johann, Krogan and several Hunters explore the caverns in search of the fabled "King of Dragons" and find what appears to be a dead end in a misty, ice-filled cavern.
On closer inspection by Krogan, the mist is revealed to be the Berserker Bewilderbeast's cold breath as they have stumbled upon her lair. Johan remarks that they will need a bigger cage as she wakes up, but doesn't initially react to their presence. Johann orders his men to subdue her with Dragon Root spears, but they either glance harmlessly off her or have no effect. Now angered, the mighty queen stirs herself and fights back, eventually unleashing a blast of ice that freezes and kills two hunters as well as sealing the remainder off from her. Johann orders his remaining men to break through the ice while he and Krogan find a different route; the Berserker Bewilderbeast glares at them through her icy barrier before retreating further in to guard her nest.
Hiccup is the first to find the egg she laid, but is forced to defend it from Johann and Krogan and nearly falls to his death with it. Hanging from the cliff, Hiccup is saved by the Berserker Bewilderbeast who fires a precise shot up at the villains, knocking them back.
As Toothless goes after Krogan, who has nabbed the egg, Johann tries to kill Hiccup with a spear but the Berserker Bewilderbeast shows herself and unleashes another icy blast. This time, she doesn't miss and Johann is frozen solid, killing him. She then withdraws further into Berserker Island and unleashes a mighty bellow, summoning virtually every dragon in the Archipelago to defend her and, by extension, Hiccup's allies, who soundly defeat the Dragon Hunters. Only Johann and Krogan actually see the Berserker Bewilderbeast. Though Hiccup is saved twice by the Berserker Bewilderbeast firing ice at his opponents, he never actually lays eyes on her. It is unknown whether she moved from Berserker Island, feeling threatened after being discovered by humans, or still lies beneath Berserker Island.
Berserker Bewilderbeast's appearance is reminiscent of that of a typical Bewilderbeast, being similar to Valka's Bewilderbeast. She is predominantly white with brown and gray markings. A noticeable difference is her smaller size compared to Valka's and Drago's Bewilderbeasts, most likely due to her being a female.
However, her most striking feature is when she unleashes her icy breath, causing a teal glow of light to emanate from her mouth just as she unleashes it. It is unknown why she possesses this feature, if it is unique to her or other female Bewilderbeasts.
Not much is known about the personality of the Berserker Bewilderbeast apart from the fact that she prefers to avoid difficult situations and prefers to use her telepathic powers to send a distress call to all of the dragons as means of defense, most likely to avoid revealing herself. She was showed some degree of irritation when Johann, Krogan and some Dragon Hunters attempted to capture her, but instead of eliminating them with her icy breath as most members of her species would, she merely roared at them and sealed off the entrance, preventing them from reaching her.
She apparently laid an egg, but seemed to show little concern for the egg's safety or care, and did little to prevent Hiccup, Krogan, and Johann from handling it. She then showed some degree of parenting by attempting twice to freeze Johann when he tried to retrieve the egg from Hiccup's hands, and then abandoned the cause, either giving up on the situation because she couldn't reach them or because she trusted Hiccup and Toothless could properly care for her unhatched offspring.
Not much is known about Johann's relationship with the Berserker Bewilderbeast, only that she found his presence irksome after he unsuccessfully tried to imprison her with his men. Johann later gave up on trying to capture the Bewilderbeast after she successfully escaped their first encounter and her size proving too much for the Dragon Hunters to handle. Instead, Johann tried to kidnap her unguarded egg. The Berserker Bewilderbeast then proceeded to attack Johann with her icy breath twice to prevent him from stealing her egg, successfully encasing Johann in ice, thus killing him on her second attempt.
The Berserker Bewilderbeast's relationship with Krogan is similar to that of Johann's. The only difference being Krogan was more cautious when around her. After being struck with the Berserker Bewilderbeast's ice along with Johann, Krogan successfully managed to steal the egg and tried to escape, only to be stopped by Toothless. After the battle Krogan returned to Drago Bludvist to report his failure to capture the King of Dragons, only for Drago to order him killed.
Hiccup never actually sees the Berserker Bewilderbeast. However, his life is saved by her twice just before Johann tries to run him through with a spear. It is unclear if she understood that Hiccup was trying to help her and the egg, or he merely was not in the direct line of fire.