A bonus dungeon is a location in a Dragon Quest game which is accessible after completing the main storyline of the game. It is considered additional bonus content to the main content of the game and typically features special rewards, challenges, or easter eggs. The concept of a bonus dungeon first appeared in Dragon Quest V, and has appeared in each installment and remakes since.
Both the Super Famicom and Game Boy Color versions contain a bonus dungeon that is accessible from the Castle of the Dragon Queen after defeating Zoma. A lone Baramonster appears at the end of the dungeon, which in turn leads to Cloudsgate Citadel (Zenithia). The dungeon is a cave made up of layouts from the previous dungeons including:
- The 2nd and 5th floor of Cave of the Necrogond
- The 1st floor of Cave Northeast of Jipang
- The 2nd floor of the Pyramid (the stairway on the upper righthand area leads to a room modeled after the basement of Tantegel Castle)
- The 2nd floor of the Underground Lake
- The 1st floor of Necrogrond cave
- The prison room of Manoza Castle
- The center of a Monster Arena.
In the Game Boy Color version, in Leiamland the spot that was occupied by Ramia will become a portal to the Ice Cave, a second Bonus Dungeon. The entrance, will become available after making a wish to Xenlon to open it.
The remakes of the fourth title introduced a new dungeon and chapter. Located beneath The Azimuth, this immense and strange dungeon featured many creatures from later games and culminated with the bizarre bonus bosses Chow Mein and Foo Yung. After defeating them the first time, a side quest starts that enables the player to resurrect Rose and recruit Psaro as a party member. The bonus bosses can be fought again to obtain special gear exclusive to Psaro.
The first true bonus dungeon of the series is Estark's Labyrinth, which contains the superboss Estark as a cameo from IV. In all remakes, after defeating Estark a special T'n'T board aptly named "Stark Raving Mad" can be played; if the player manages to reach the finish, it unlocks two unique and powerful monster companions.
After defeating the final boss, if the party have between them reached the fifth rank of every single vocation, including the secret Dragon and Liquid Metal Slime, then they can enter the Fungeon through the back chamber of Alltrades Abbey in the Upper World. Besides taking the heroes to a grim mirror of Weaver's Peak, with some of the rarest items for purchase, the Fungeon is home to one of the most powerful monsters in the entire series, the demon lord who destroyed Castle Graceskull, the great and terrible Nokturnus. If the heroes can defeat him within 20 turns, the player will be treated to a very special ending variant of the final battle against Mortamor.
The seventh title features two bonus dungeons to spelunk once the demonlord is vanquished: the Cave to Another World and the Cave to Yet Another World. Both of these extras require their own sets of fragments to be amassed and assembled in the Shrine of Mysteries, and consist of recycled areas explored earlier in the game with more powerful monsters and greater treasures. At the end of the first bonus dungeon is The Almighty, who will offer the party a friendly bout after congratulating them for their undertakings. The fight at the end of the second dungeon is the quartet of elementals: the Earth Spirit, Fire Spirit, Water Spirit, and Wind Spirit. The aggressively defend the humble dwelling of The Almighty and will not even allow the warriors of eden to approach it.
The bonus dungeon is the Dragovian Path, which features a sidequest that uncovers the Hero's heritage. After completing the Dragovian Trials, a new ending occurs after defeating the final boss again. In the 3DS version, there are two new bonus dungeons, the Altar of Wroth and Memories Lane. The Altar of Wroth can be accessed after the destruction of Neos and visiting the depths of Dragon Graveyard to talk to Golding's ancestor. Memories Lane is a secret dungeon that can be unlocked by the Lord of the Dragovians as a prize for completing one of the Dragovian Trials.
The Tower of Nod becomes available after completing the main storyline and obtaining the Starflight Express. Furthermore, the extensive grotto system functions as sort of bonus dungeon in and of itself, with the absurdly powerful bosses waiting within rivialing those seen in previous games--not to mention those old foes themselves in the form of Legacy Bosses.