As Ventus travels between worlds, he is attacked by a mysterious Unversed. He fights it off, but the Metamorphosis flies towards the Grand Councilwoman's spaceship. Ventus trails after it and pursues it throughout the ship, meeting Experiment 626 along the way. Ventus eventually catches up to the Metamorphosis in the engine room, where it had been causing large amounts of damage, and teams up with Experiment 626 to defeat it.
The Metamorphosis is a large, jellyfish-like Unversed with pale blue skin that fades to white at its tentacles, and a body adorned with silver linings. It has a sleek body made primarily of three large tentacles, each with arrowhead-shaped tips and light blue diamonds. Its lower body condenses into a twisted, black tail. The Unversed emblem adorns the front of its lower body, while its circular, black face is on the tip of its upper body. Its red eyes are set in a sorrowful expression.
In Kingdom Hearts Birth by Sleep Final Mix, the Metamorphosis's skin becomes white, which fades to black at its tentacles. The lining on its body is now purple, with turquoise diamonds on each tentacle. Its lower body and tail are also purple and fade to black toward the bottom.
Metamorphosis is the process by which something transforms into another form, which this Unversed does.
 First battle
The first battle against the Metamorphosis is a unique one indeed, one that is not carried out in any other boss battle in Ventus's scenario. Similar to Sora's battle with Xemnas's Dragon Form in Kingdom Hearts II, the command menu will change only to host four commands. will let Ventus use the Charge technique, will allow him to jump, will allow him to put up a barrier, and will allow him to attack the Unversed boss. As this is a change in the battle system, no Command Styles may be used. While the Focus and D-Link Gauge will be filled and ready for use, they too cannot be used. The player does not need to deplete the Metamorphosis's HP to zero in this battle. All they must do is attack until the battle ends automatically and survive.
At the start of the battle, immediately Lock-On to the boss. The player can then begin to attack the foe with the Air Gyre or Charge commands. Both of these attacks have a relatively long range, and they have a strong chance of hitting. The Metamorphosis will only charge at Ventus or spin in place to damage him with its tentacle-like arms, Barrier able to defend against both attacks if its usage is timed correctly. Simply continue the onslaught and use Barrier if necessary, and the battle will end rather quickly.
 Second battle
The second all-out battle against the Metamorphosis is unique, as well. Not only will the player have Experiment 626 as a party member, but they also will have to prevent the ship's engine from taking too much damage. Ventus can use Shotlocks and Command Styles but not D-Links due to 626's presence. Instead, the player should use 626's presence to their advantage, as he is the key to being triumphant in this battle. He also doesn't need to be healed, automatically reviving if his HP hits zero.
At the start of the battle, again Lock-On to the Metamorphosis. Begin hitting it with aerial combos, but do not get cocky. The Unversed will soon begin to fly out of attack range, certain Deck Commands only able to hit it at this point. Should the Metamorphosis latch onto the glowing column in the center of the battlefield, the yellow bar indicating the engine's damage on the top-left of the screen will begin to fill (if the bar fills at this or any point during the battle, Ventus will lose and have to retry the boss fight). Bombard the Unversed with Ventus's strongest long-range or aerial combos, attacks such as Tornado Strike recommended (press when the on-screen prompt appears each time for maximum results). In terms of its counter attacks, the Metamorphosis will only rush at the player or spin like a top towards Ventus, its arms slashing as it does. It can also spin around while shooting lightning from its arms, and it can also fire a laser from its tail.
Eventually, the Metamorphosis will vanish, Ventus's only way to track it being the Lock-On feature. Ventus will not be able to stay Locked-On, however, as the Unversed will vanish and reappear far out of attack range on the very top of the illuminated column. This is the time for the player to perform a special Deck Command with 626 called "Astro-Strike", one that will launch him to the Metamorphosis's location. While the Metamorphosis will have all ready increased the engine's damage level a great deal, 626 will detach it from the column, allowing the Unversed to be attacked once more. To surmise, the key to winning this battle is staying away from the boss, only approaching it to perform Ventus's fast-paced combos. Otherwise, long-ranged attacks should be favored. Use Astro-Strike when necessary, and the battle will end in victory.