From the nighttime views of the island under attack in Shell Shocked, Part 1, Berserker Island appears to be a very rocky, bare island. It is very 'vertical', having a tall central mountain with very little spread of hills of fields around it.
More detail is revealed in "Something Rotten on Berserker Island". The rocky central island is surrounded by significant sea stacks. The island has trees and grass, and a large amount of white-flowering bushes closer to the village proper. The interior of the island has a "maze of caves", in which ice structures can be found, which were created by a Bewilderbeast. The caverns are so extensive, the mountain is practically hollowed out.
In "Something Rotten on Berserker Island", Dagur has returned as chief of the Berserker Tribe, and is rebuilding the village. He accepts Savage into their midst, but Savage betrays the Berserkers by staging a coup.
All Dragons on the island are seen to act strangely - getting disoriented and not listening to their riders - when getting close to Berserker village, but not above a certain altitude. Given the strange ice-like formations in the underground caves of the island, it is entirely possible that a Bewilderbeast is present on the island. This also may help to explain Heather's Dragon Eye Lens and its relation to the King of Dragons' mentioned by Johann in "Sins of the Past".
Berserk Island is seen again in the episode, "In Plain Sight", when Dagur tried to get back a Dragon Eye lens from a childhood bully, Ansson. The Berserker Docks are seen, indicating a fishing industry on the island. The "Thunderfish", a Scauldron, inhabits some territorial fishing waters of the Berserkers, but Ansson and Dagur are able to drive it off.
Berserker Island is revisited at the end of "King of Dragons, Part 1". Johann and his cohorts descend upon the island, having determined the 'King of Dragons' resides there. Dagur had just commented that everything was going so well on the Island, that he was bored.
The final battle of the series takes place on Berserker Island in "King of Dragons, Part 2". The Riders, Berserkers, and other allies clash with Johann's and Krogan's forces. Hiccup, Johann, and Krogan battle over possession of a Bewilderbeast egg inside the island, as well as an adult Bewilderbeast.
Though it doesn't appear in game, Dagur the Deranged mentions Berserker Island during a quest in the expansion Rise of Stormheart.