The Citadel is a location first introduced in Monster Hunter Rise: Sunbreak. It is a massive area featuring 3 sub-zones, similar to several locales in Monster Hunter: World, with each zone featuring unique environments. The main areas of the Citadel are a large ruined castle, a swamp filled with thick mud, and a cavernous snow area filled with massive ice crystals. Each of these areas are dominated by one of the monsters known as the Three Lords; Malzeno haunts the castle, Garangolm lurks in the swamp, and Lunagaron stalks the icy caverns. Other powerful monsters can be found in each of these areas as well.
Source: http://monsterhunter.wikia.com/wiki/Monster_Hunter_Wiki
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