Vanitas Remnant

A being called the Vanitas's Sentiment (ヴァニタスの思念) appears as a secret boss found in the Keyblade Graveyard in Kingdom Hearts Birth by Sleep.

Kingdom Hearts Birth by Sleep
Before attacking Terra, Ventus or Aqua, its battle must first be accessed by entering a dark portal with a picture of the charm found on the end of the keychain found on Vanitas's Keyblade (similar to encountering an Absent Silhouette). The sentiment drops a Hidden Gem (秘められし魔石) when defeated.

Strategy
While the Vanitas Sentiment has significantly less HP (one bar) than some bosses previously fought, it is important not to underestimate it. The Vanitas Sentiment uses a skill-set similar to the true Vanitas, but with more powerful versions of Vanitas's attacks and the ability to heal itself whenever the player tries to do the same, excluding healing by potions. Each of these tactics have the potential to instantly drop the player's HP into the "Alarm Zone", thus the boss should be approached with caution.

Some differences in the Vanitas Sentiment's moveset include the fact that its standard behind-aerial attack can hit multiple times, time before the attack when the boss freezes in place significantly shorter. The fireballs it shoots fire at a much faster pace. The Vanitas Sentiment also has a fair share of new tactics in its arsenal, however, able to dash around in the shadows before several dark, crystalline and floating blades attack the player from both sides. When the boss holds its blade above its head, the Keyblade it holds powers up, leading to several powerful slashes, the combo finishing with a long-range slash in the shape of an "X" that will home in on the player unless it comes in contact with an obstacle beforehand. It may also surround itself in darkness, "shadow" versions of itself performing aerial combos. Not only this, but the Vanitas Sentiment will then proceed to power up, surging with energy, attacking its opponent from beneath. It should be noted that several of his attacks can cause the player's view to be obstructed by darkness for a certain amount of time, making it easy to be hit by any follow-up attacks he may use. The Vanitas Sentiment also has the ability to perform a Shoot Lock, firing a massive laser with a relatively long range directly in front of it. The attack is easily avoidable, however, and it also leaves the boss open for attack.

It is best that the player keep a continuous hit-and-run pattern going during this battle. Victory is simply determined by the proper timing of Guard and attacks, as well as the equipment of proper Deck Commands.

Video
http://www.youtube.com/watch?v=Y6NC0ejfjoE http://www.youtube.com/watch?v=1DtDEJ4-wcE

Trivia

 * The Vanitas Sentiment shares many similarities with the Absent Silhouette and the Lingering Sentiment in Kingdom Hearts II Final Mix. It's possible that the Vanitas Sentiment is a reference to a character to a new Kingdom Hearts game (possibly in the future game referened in Reconnect. Kingdom Hearts).
 * The Vanitas's Sentiment is chronologically the first Absent Silhouette to exist. Whether this means the Absent Silhouettes encountered in Kingdom Hearts II Final Mix are Unversed or not as the Sentiment is has yet to be revealed.
 * The Vanitas Sentiment is also the only boss in Kingdom Hearts Birth by Sleep that is able to perform a version of the Shoot Lock.
 * The Vanitas Sentiment first appears before Terra, Ventus, or Aqua from a sphere of dark energy that later morphs into the latter's body, much as Vanitas did upon his creation.
 * The Vanitas Sentiment is also the first secret boss to speak during battle. Both Xemnas and the Lingering Sentiment were mute during their boss fights in their respected games.