Here is the full list of all 55 Ancient Dungeon achievements.
Damage The Beast with the Mercenary's Armament.
Acquire four items from shops during a single delve.
Defeat a Mad Merchant.
Damage The Beast for the first time.
Defeat a boss enemy with a single strike.
Traverse The Infested Dungeon without taking any damage.
Strike enemies with a ranged weapon twenty-five times in a row without missing.
Use The Blacksmith's Anvil to augment a weapon.
Acquire three highly powerful items during a single delve.
Explore two hundred fifty rooms within The Ancient Dungeon.
Deal at least twenty-five damage with a single strike.
Defeat a Possessed Armor.
Discover a secret room in the ceiling.
Discover a secret room in the wall.
Enter The Ancient Dungeon for the first time.
Use a Contracting Orb to shrink yourself.
Overcome a Challenge Chest's trial.
Defeat a Chest Mimic.
Damage The Beast with the Assassin's Armament.
Defeat one hundred aberrant enemies.
Destroy a Sacrifice Statue.
Raise your merchant discount to 60%.
Destroy one thousand pots.
Discover (almost) all items.
Carry more than two hundred fifty coins during a single delve.
Escape The Ancient Dungeon without consuming any food items.
Acquire more than five full hearts of health.
Win big on The Merchant's slot machine.
Carry more than twenty keys during a single delve.
Eat every type of food item during a single delve.
Defeat a Golden Slime.
Escape the dungeon without obtaining any upgrade items.
Traverse The Overgrown Gatehouse without taking any damage.
Traverse The Forgotten Library without taking any damage.
Traverse The Luminous Mines without taking any damage.
Escape The Ancient Dungeon after re-entering once.
Escape The Ancient Dungeon after re-entering three times.
Escape The Ancient Dungeon after re-entering twice.
Lower your merchant discount to -100%.
Damage The Beast with the Mercenary's Armament in hard mode.
Damage The Beast with the Assassin's Armament in Hard Mode
Use a Bandit Box at least twenty times during a single delve.
Escape the Ancient Dungeon without entering a shop.
Escape the Ancient Dungeon in under twenty minutes.
Fully augment the Assassin's Armament.
Fully augment the Mercenary's Armament.
Traverse The Beast's Cradle without taking any damage.
Consume a food item made of gold.
Throw a dragonfruit into a pit.
Complete the delve of every Sealed Soul.
Complete 3 unique sealed soul delves
Challenge and destroy a boss with only a half-heart of health
Open 250 chests
Escape The Ancient Dungeon with a difficulty modifier of 2.25
Escape The Ancient Dungeon with a difficulty modifier of 3.00