Be careful! Sometimes monsters lurk inside treasure chests!
- —Adventuring School in Final Fantasy VI
Monsters-in-a-box (敵入りの宝箱, Teki Iri no Takarabako?, lit. Treasure Chest Containing Enemy) are enemies that appear in treasure chests. After the character opens the Chest, the phrase "Monster-in-a-box!", "Monsters!", or similar is displayed, and a battle begins with an enemy. Usually these enemies are stronger than average foes, and can even match a boss's strength. Monsters-in-a-box were series staples up to Final Fantasy VIII where they do not appear, and only return to the series in Final Fantasy X.
Previous to the use of monster-in-a-box treasure chest starting in Final Fantasy II, the original Final Fantasy surrounded treasure chests with fixed encounters to protect boxes. As a precursor to the monster-in-a-box, enemies fought through this method still had the tendency to be stronger than average. In the later games of the Final Fantasy series, Mimic enemies act as a replacement which appear as boxes and morph into an enemy.
In Final Fantasy II, Monsters-in-a-box are very common, appearing in almost every dungeon. Unlike later installments, the item is gained before the monster is fought. All types of dragons appear as Monsters-in-a-box in some later dungeons and in the last dungeon, Pandaemonium, several of the game's most powerful armor pieces can be found in boss fights with Monsters-in-a-box.
Early in the game in Castle Sasune, the party must defeat a monster-in-a-box, a Griffon, to obtain the Wightslayer. In Eureka, a Ribbon is obtained after opening a treasure chest and defeating the Ninja within. During the final dungeon of the game, there is a crossroad of four paths. Each path has a chest along the way, with a Xande's Clone guarding a Ribbon.
Many monsters-in-boxes can be fought in Final Fantasy IV. After opening a box, a message saying "Monsters!" will appear and the fight will initiate. After winning the battle, the item contained in the chest will be received.
In the DS version, the word "Monsters!" has been replaced by "Enemy Ambush." If there is an item, the item will be received, followed by the term "Enemy Ambush!". For example, in the Giant of Babil, the message box will say:
Monster-in-a-box appears when the player opens certain Treasure Chests, and is commonly extremely strong for the location. Once the enemy is defeated, a message is displayed and the item is received. The superboss Shinryu appears as a monster-in-a-box in the Interdimensional Rift, which guards the strongest weapon in the game, Ragnarok. In the GBA Remake, Neo Shinryu is encountered the same way, guarding the Ultima Weapon.
Monsters-in-a-box only appear once in the clock puzzle room of the Temple of the Ancients. They do not give items.
In the Omega Ruins, there are areas where three to five treasure chests are grouped together. Some contain treasure, while others initiate a battle. After the battle, the other nearby treasure chests will disappear. It is random which treasures contain battles.
The treasures contain the following items:
- L4 Key Sphere
- Defending Bracer
- 3x L3 Key Sphere
- Defending Armlet
- 2x Friend Sphere
- L4 Key Sphere
- Phantom Ring
- Cactuar Wizard
- 2x Warp Sphere
- 99x Warp Sphere
There are several monster-in-a-box chests in the Fiend Colony. The battles that initiate are against Vermin-type enemies.
Patch 2.1 added the Treasure Hunt system, which uses a monster-in-a-box mechanic. The buried treasure coffers have a trap that will release a "powerful musk" when opened that summon monsters. These creatures must be defeated in order to claim the goods contained within.
The sewer has two trapped treasure chests which contains monsters. The player can farm them for high level meat if they want, but the player doesn't get any items from these treasure chest. The player can repeatedly open them infinitely if they wish to fight the monsters again.