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.
The Void Wizard boss is located on the second floor.
The First Floor
The Second Floor
The Void Wizard won't attack you unless you attack him first. He doesn't respawn after being killed.