The Iron Imprisoner is an Unversed that is found in Kingdom Hearts Birth by Sleep. It is one of the bosses at Mirage Arena. When it is first fought, it is chained and imprisoned in a gibbet, but it gradually releases itself over the course of its battles.
The Iron Imprisoner is a large, armored Unversed with four different forms. In its first form, the Iron Imprisoner I's lower body is locked in a black gibbet with silver highlights and several black, diamond-shaped gems set in it. Its arms, which are covered in black armor with two blue gauntlets, are bound by four red chains. Only its head, encased in a helmet that is light grey on its right half and grey on its left half, is free to move. It has jagged, black horns on either side of its helmet, though they are not of the same shape. The right horn is somewhat straighter and more horizontal, while the left horn hooks upward sharply. Its eyes are red, set in an angry expression, and slightly uneven.
The Iron Imprisoner II is identical to its first form, save for two significant changes. The Iron Imprisoner's chains are gone and its arms are free, making its gauntlets and red collar clearer. It is also now wielding a large, red and black hammer with spikes on its head. The handle of this hammer is tan and the pommel is a black spike. The head of the hammer can conjure fire.
The Iron Imprisoner III is now free of its gibbet, exposing its grey legs. Both of its feet sport black bands with blue highlights on them. Its collar is much smaller and the grey breastplate it wears is now clearly visible. Its hammer has also filled out.
The Iron Imprisoner IV has shed the last of his shackles. Its black, angry face has been revealed from under its iron mask, and the silver bands on its upper arms have been removed. Its boots and gauntlets have been removed, revealing its natural hands and feet. Most importantly, its hammer has grown even larger, and is more intricately detailed. Its lower legs are light grey and the tips of its feet are red. Its hands are red and black.
On each form, the Unversed insignia is on its back.
The former part of the Iron Imprisoner's name resembles its metallic appearance, while the latter part represents its ability to imprison players in cages. It may also reference the Unversed's own imprisonment in its chains and gibbet.
For the Iron Imprisoner's Final Mix color scheme, its body is now purple with fuchsia arms and legs. The collars on its arms and legs are now green. Its lower body is now locked in a black gibbet with gold highlights, bound in gold chains. The hammer handle is now silver, with a gold spike, and the hammer itself is purple with green highlights. The Iron Imprisoner's helmet is half black and half white.
The Iron Imprisoner is one of the more powerful bosses in the game. The player should have its strongest Shotlock equipped, but should use the basic form of its dash ability, which will be used to avoid damage. It is also vital that the player equip the Once More and Second Chance abilities, and have several Cure-type Commands in its deck. Vanish, which keeps the Iron Imprisoner from targetting the player, is also a very good idea.
Note that for all forms, while playing as Aqua, Firaga Burst will damage and remove the cage when it is summoned.
 Iron Imprisoner I
In its first form, the Iron Imprisoner is chained and trapped in a gibbet. It attacks mainly by swinging the gibbet at you or spinning around like a fiery tornado. It can also entrap you in a cage or erupt out of the ground like a volcano. The best way to start the battle is to use your Shotlock to shave away at its HP bar, then attack normally. Beware when its HP is low. It shoots lasers from the bottom of its gibbet, dealing fire damage on impact.
Overall, it is the easiest of the four forms. Merely dodge the lasers and attack. Alternatively, if you wish to stop any one of its attacks and keep it immobilized, cast Firaga on it repeatedly.
 Iron Imprisoner II
In its second form, the Iron Imprisoner's arms are free, and it holds a massive hammer. It can also spin with the hammer outstretched, dealing fire damage on contact. It retains all of its attacks from its first form, only that it moves slightly faster. Simply dodge, attack and use Shotlocks or cast multiple Firagas on it when possible.
 Iron Imprisoner III
In this form, the Iron Imprisoner's legs are now free of its gibbet. As a result, it is much faster, but it cannot shoot lasers. It retains its other moves so the strategy is the same. Shotlock commands are the most effective way to damage the Iron Imprisoner, and as a result the Damage Syphon ability is very useful. While it will pause periodically, the window of attack is very small, and it is safer to use the time to heal, use a Shotlock or cast multiple Firagas.
There are three safe windows of attack: after the Iron Imprisoner summons its cage, immediately after it emerges from the ground, or when it is very far from the player. The first two instances are safe for the player to use Shotlocks; the last instance requires multiple Firagas to subdue the Iron Imprisoner.
Evading the cage is difficult unless the player is executing a finishing command or a Shotlock. If timed correctly, one can avoid the cage by attacking the Imprisoner with a combo.
There is a foolproof way of dodging the cage and preventing it from imprisoning the player: when the cage is still in the ground following the player, dodge in the opposite way in which the cage is going. That way, the minute the cage meets the player, it will rise from beneath the ground and no longer chase the player around. The player, having dodged, will be free to move. This move, however, is not recommended when the Iron Imprisoner is down to its last two bars of health, due to its more vigorous attacks.
 Iron Imprisoner IV
In its final and most powerful form, the last pieces of miscellaneous binding have been removed, enabling its full moveset. The cage it traps you with is now on fire and will damage you on contact. It also has an attack where it sucks you in close before releasing a large explosion. During another new attack, it will possess the room, turning it red and causing the tiles to fly off and attack. The strategy for the third form is also effective here, and the boss can still be stunned by a successful Firaga or Sonic Blade. However, because of the Iron Imprisoner IV's increased strength and ferocity, the player should be more cautious. When the boss possesses the stage, the player can either choose to stay in one place and block constantly to defend against the flying debris, or simply Dodge Roll, Cartwheel, or Slide around to evade damage. It would also be wise to equip a Vanish command, which can be bought in the medal store, as when the character is invisible, the Iron Imprisoner mostly avoids them, giving you a chance to heal or use a shotlock.