Vanitas Remnant

Vanitas's Sentiment appears as the secret boss found in the Keyblade Graveyard in 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. After entering the portal, Vanitas's Sentiment forms from a sphere of dark energy, similarly to Vanitas's creation. Vanitas's Sentiment drops a Hidden Ore when defeated, then disappears in the same way he appeared.

Strategy
Vanitas's 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 has the potential to instantly drop the player's HP into the "Alarm Zone", thus the boss should be approached with caution. It also has some attacks that Vanitas could only do with the X-Blade, both complete and incomplete.

For the most part, Vanitas's Sentiment will start the battle by surrounding itself in a dark ball of energy and sending multiple shadow versions of itself after the player. While these are extremely difficult to dodge, it is not impossible. Hiding behind a pillar may help, but not always. If one of these shadows hits the player, they will be temporarily blinded and left open to further attack, which will most likely finish the player off, as the first hit usually sends them into the alarm zone if lucky. A rather risky way of avoiding this is quickly hitting Vanitas's Sentiment just before it can begin the attack, then dodging away before it begins a different attack. In it's standard moveset, it has two melee attacks: one where it will strike at the player and then send an arch of energy after them, which will follow the player for the next few seconds before fading out. Beware not to be hit by this, otherwise the player will be confused and very open to another attack. The second one involves the Sentiment striking at the player once, striking the ground, creating a wave of energy in the form of an "X", then striking at the player again, and ending it by leaping into the air and firing an "X" shape of energy at the player, which is easily dodged. This last one will leave the Sentiment open for a couple of hits, but the player must calculate their moves carefully or it can be dearly costly. If the Sentiment is struck once too many times, it will quickly retaliate with an ice-based attack which could kill the player instantly.

The rest of the Sentiment's moves are not really that hard to dodge; it is finding the right moment to hit it than can be difficult. Occasionally but not very frequently, the Sentiment will unleash other attacks like teleporting itself around the player and attacking. Depending on the player's level, this attack can be countered with a guard. Another attack is diving underneath the ground and following the player from below, suddenly bursting out and unleashing fireballs around itself. This later one can be dodged by quickly dodge rolling just when the Sentiment's mark is underneath the player. Another attack is just firing a fireball at the player which will explode into several smaller balls and home in on the player, much like the original Vanitas's attack, only the fireballs will follow the player longer and will hurt more. Probably the easiest attack to dodge and then take advantage of is the Sentiment's "Shoot Lock", indicated by several red dots appearing on the player and the Sentiment firing a beam of energy. This beam can kill the player if not careful, but the player can use this time to work their way around the Sentiment and strike it a few times before the attack ends. Another lethal but easy to dodge attack is when the Sentiment dashes around in a circle and leave spikes of black energy which will suddenly join and instantly kill the player should they be in the wrong place.

During the fight, it is highly recommended that the player equip a deck solely with potions for healing, as using Cure spells will prompt the Sentiment to heal itself as well, making life hard for the player. Do not equip special commands or use Shoot Locks, as they will have little to no effect and could even leave the player open to attack for even the slightest moment.

Stick to this strategy and the victory will be the players'. Basically the trick is to keep a hit and run strategy. While the Sentiment has only one life bar, it is important not to underestimate the fight, which could take even longer than one against the Lingering Sentiment, so above all, patience is a must.

Video
Video:Kingdom Hearts: Birth by Sleep Vanitas's Sentiment Boss Video:Kingdom Hearts : Birth by Sleep - Vanitas Lingering Sentiment Defeated (Whole Fight with Aqua) Video:Kingdom Hearts Birth By Sleep - Terra vs Secret Boss (Lingering Sentiment Vanitas)

Trivia

 * Both Vanitas's Sentiment and the Lingering Sentiment, a similar boss in Kingdom Hearts II Final Mix, appear in the Keyblade Graveyard.
 * Vanitas's Sentiment is the first hidden superboss who actually speaks, as opposed to the Enigmatic Man and the Lingering Sentiment, who were completely mute during battle.