A Nation Dungeon is a special type of time-limit dungeon that can only be opened and entered by a Nation Officers or Leaders. It's usually comprised of multiple floors with many mobs to kill, and since requirements for clearing tend to be rather high, squad preparation is a must. Like Individual Dungeons and Guild Dungeons, there's a cool down period once a dungeon is closed, regardless of it ending in victory or defeat.
Some nation dungeons cannot be attempted with a full nine character party, and so some mercenaries must be disabled by each player.
Nation Dungeons also drop Star Stones.