Vanitas Remnant

Vanitas's Remnant makes its first and only appearance as a secret, optional boss in Kingdom Hearts Birth by Sleep. After the player clears the Final Episode, a crest similar to an Absent Silhouette portal will appear in the Badlands area of the Keyblade Graveyard. Simply approach the portal and select the "That's a danger I'm willing to face" option to initiate the battle. Defeating Vanitas's Remnant will earn the player a Secret Gem in the Japanese version of Kingdom Hearts Birth by Sleep, and the Void Gear Keyblade in North American and European versions. The Void Gear will also be the player's prize for defeating the boss in Kingdom Hearts Birth by Sleep Final Mix. Vanitas's Remnant also must be defeated if the player wishes to challenge the even more powerful Mysterious Figure.

Story
Terra, Ventus, or Aqua returns to the Badlands to find a mysterious sphere of dark energy hovering in the air in front of a large pillar of rock that was not present during their last visit to the area. They ignore the dark power felt emanating from the crest, accepting the challenge it presents. Vanitas's Remnant then takes shape behind them from a sphere of dark energy, much like the true Vanitas did during his creation, drawing its Void Gear Keyblade and attacking. After the Remnant's defeat, it transforms back into the dark energy sphere from which it originally spawned and vanishes.

Design
Vanitas's Remnant, true to its name, is a palette swap of the helmetted Vanitas. Its smooth helmet is completely black with an Unversed insignia emblazoned on it. Its torso and legs are covered by a black bodysuit with nearly glowing white highlights on the chest. The material over its arms shifts in color from black to grey to white, before shifting to black again on the palms of its hands. Its boots are white with black soles. It also wears a white, skirt-like garment with black tips around its waist, held up by several grey straps.

The crest that must be approached by the player to begin the battle has a very similar appearance to an Absent Silhouette portal, varying shades of purple in coloration with shards of black, rock-like material floating around it. The symbol appearing on the crest displays two halves of a gear which join to form an "S", the keychain of the Void Gear Keyblade, both Vanitas and his Remnant's primary weapon.

Strategy
Vanitas's Remnant uses a skillset 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 perform with the χ-blade, both complete and incomplete.

For the most part, Vanitas's Remnant 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 Remnant just before it can begin the attack, then dodging away before it begins a different attack. In its 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 Remnant 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 Remnant open for a couple of hits, but the player must calculate their moves carefully or it can be costly. If the Remnant is struck one too many times, it will quickly retaliate with an ice-based attack which can defeat the player instantly.

The rest of the Remnant's moves are not really that hard to dodge; the hard part is finding an opening to attack it. Occasionally but not very frequently, the Remnant 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. Also, if you are a far distance away from the Remnant, it will teleport at a random interval, leaving an afterimage and perform a direct thrust at the player. Another attack is diving underneath the ground and following the player from below, suddenly bursting out and unleashing fireballs around itself. This can be dodged by quickly dodge rolling just when the Remnant'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 Remnant's "Shotlock", indicated by several red dots appearing on the player and the Remnant 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 Remnant and strike it a few times before the attack ends. Another lethal but easy to dodge attack is when the Remnant 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.

When playing as Terra, it is recommended that the player equip a deck solely with potions for healing, as using Cure spells will prompt the Remnant to heal itself as well, making the battle hard for the player, and since Terra is by far the strongest of the trio, especially with the Chaos Ripper, normal attacks will deal damage to the boss.

As Ventus, the fight becomes much easier if the player has equipped a mastered Tornado (or even two), one of the very few Magic attacks able to do considerable damage to the Remnant (but is only available for Ventus). The only problem is finding the right moment to use it successfully, one such is immediatly after being hit. Using the Tornado on the Remnant twice and then striking it normally will end the battle quickly.

As Aqua, certain Magic attacks can do damage to the Remnant, such as Mine spells, but the only problem is the possibility of filling up Aqua's command gauge with Mine hits which will trigger an unwanted Command Style and leave Aqua open to a possibly fatal attack from the Remnant. Using the Hat Trick Ice Cream to get Aqua in the Spellweaver Command Style and using its finisher twice will end the battle quickly. If the player is at a high level and is using a strong Keyblade, then the same strategy for Terra can be used.

It is also recommended that the player complete the "Villains' Vendetta" challenge in the Mirage Arena to obtain the Ultima Weapon Keyblade for the highest Attack and Magic boosts possible. Do not use Shotlocks, as they will have little to no effect and could even leave the player open to attack for even the slightest moment. If desperate, the Shotlock may be used to protect oneself from the Remnant's shadow attack, although it is very risky, as the end of the Shot Lock will leave the player vulnerable for a moment which could very well be when the Remnant strikes again, possibly fatally. Synthesizing and equipping Once More and Second Chance can help in sparing the player from the Remnant's unforeseen attacks.

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

When defeated, Vanitas's Remnant then disappears in the same way it appeared. It drops a Secret Gem in the Japanese realease of the game, and the Void Gear Keyblade in the English release.