Large Dungeon

If you make 5 trades with one Wandering Trader, it will trade you an Obsidian Key for an enormous amount of emeralds. This rare item would be the only way to enter any Boss Dungeon. These underground structures would have traps, mobs, and loot all over it's many rooms, and in the last one would be a giant zombie and a chest with a unique item. When you defeat the boss, the chest appears and the unique item appears. It could be a legendary sword, some powerful potion, even a leash for monsters!