The Northern Markets first appeared in the episode, "Saving Shattermaster".
|“||Once an unspoiled island, the Northern Markets now house an unsavory port teeming with pirates, weapon mongers, and dragon trappers.||”|
|— Hiccup's map|
This island appears to have a bustling market of sketchy Viking traders and weapon-smiths. The land outside the markets, however, appears to have healthy grass, trees, and other vegetation, on a hilly and mountainous island. There is at least one area along the shoreline suitable for docking ships filled with all the possible items merchants would want to sell to anyone who visits.
The episode, "Saving Shattermaster", opens with the Defenders of the Wing attempting to buy weapons at the Northern Markets. There is a shadowy figure lurking there that appears to be working with the Dragon Hunters. Upon investigation, the Dragon Riders discover Dagur the Deranged is at the Northern Markets, posing as a friend of the Hunters. His real goal, however, is to save Shattermaster from slavery.
In the episode, "Sins of the Past", a bit more of the market is seen, as Snotlout Jorgenson walks through it in search of chicken feed. He meets a Chicken Feed Salesman. There is also a seedy tavern on the island, where Heather meets Harek to find out information about her father.
In "In Plain Sight", Hiccup and Johann go to the Northern Markets to find gemstones needed for the completion of the Dragon Eye II. While searching for the last ones, they go into the Sandbuster's lair where Johann reveals his true intentions to Hiccup. After a short fight, Hiccup and Toothless leave the island.
The Northern Markets is not seen, but mentioned once in the September 2017 Gauntlet Event. Apparently, according to Fishlegs, the Twins visit the Market and purchase a Skrill egg, in order to start a singing band.