Iron Imprisoner

The Iron Prisoner (アイアンプリズナー) is an Unversed that is found in Kingdom Hearts Birth by Sleep. Although it is a boss, it only appears 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 four forms it takes are known as Iron Prisoner I, Iron Prisoner II, Iron Prisoner III, and Iron Prisoner IV.

Design
The Iron Prisoner is a large, powerful Unversed whose entire body is clad in metal armor. It wields a fiery hammer as large as the creature itself. As it levels up, the armor's features become more pronounced. In its first form The Unversed's arms chained and its legs trapped in a gibbet. Its helmet is horned and sports fierce red eyes. Its second form is the same as the first albeit its arms are free and it wields its hammer. Its third form is the same as the previous two, the only difference is that its legs are free, allowing more mobility. Its fourth and final form is extremely different than the previous three. Its body sports a darker shade of grey, the eyes have a fiercer design than the others.

Abilities
The Iron Prisoner is one of the strongest Unversed bosses in the game. It wields its giant hammer in battle, and the element of fire to enhance its attacks. It can erupt out of the ground with the force of a geyser. It can also entrap players in cages leaving players at its mercy. It can also spin around the arena while coated in a fiery aura, burning players on contact. In its first and second form it can shoot lasers from the bottom of the gibbet to burn players. In its fourth form it can suck players in to deal damage, and possess the room turning it red and causing the tiles to fly off and attack players. It mainly moves around the arena by levitating or teleporting.

Strategy
The Iron Prisoner is one of the most powerful bosses in the game, second only to Vanitas's Sentiment. Therefore it is recommended the player be at a high level, have the Ultima Weapon keyblade, and have the strongest Shoot Lock available: Ultima Cannon for Terra, Shiny Bloom for Aqua, and Pulse Bomb for Ventus. The strategies for each form are below.

Iron Prisoner I
In its first form, it's 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 Shoot Lock 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. But overall, it is the easiest of the four forms. Merely dodge the lasers and attack.

Iron Prisoner II
In its second form, nothing much has changed except for the fact it now uses its massive hammer to deal large damage. It can also spin with the hammer outstretched, dealing fire damage on contact. otherwise, it retains all of its attacks from its first form. so simply dodge, attack and use Shoot Locks when possible.

Iron Prisoner III
Its third form is the same as its previous two incarnations. With the only obvious difference that its legs are no longer bound in a gibbet. It is now faster than the other two forms, but it cannot shoot lasers. It retains its other moves so the strategy is the same.

Iron Prisoner IV
The boss's final form, and its most powerful one yet. It retains its old moves with some new ones. 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. When its HP is low it will possess the room, turning it red and causing the tiles to fly off and attack.

Etymology
The former part of the boss' name resembles its metallic appearance, while the latter part represents how it is chained and imprisoned in a gibbet.