Dungeon

You can access Dungeon through the Dungeon Entrance island. Dungeon has two floors. Enchanted Zombie, Enchanted Zombie Archer, Enchanted Crawling Zombie and Void Apprentice spawn on the first floor.
You can rescue and recruit the Mage Settler here, multiple can spawn and be recruited in a single dungeon.

The Void Wizard boss is located on the second floor.

Book can be found in the Dungeon bookshelves, and Void Shard is dropped by Void Apprentice and Void Wizard.

Easter Eggs

The spawn of 2147483647, 2147483647 is on the exact point of a dungeon entrance and thusly removes the dungeon entrance.

Dungeon Entrance

The Dungeon Entrance is guarded by Vampire or Void Apprentice. They do not respawn after being killed.

The First Floor

The first floor of the dungeon can contain the following possible items, found in treasure chests scattered throughout:
Void Spear, Void Boomerang, Lightning Hammer, Mobility Cloak, and Mesmer Tablet.

The Second Floor

The Void Wizard won't attack you unless you attack him first. He doesn't respawn after being killed.