Keyblade

The Keyblades are mysterious weapons that are heavily featured in the Kingdom Hearts series. Wielded by the main character, Sora, as well as a number of other important characters, the Keyblade is a main part of the battle between Darkness and Light.

Powers
"The Keyblade. A truly marvelous weapon...were it only in more capable hands." -hooded Xemnas

As the name implies, in combat, Keyblades are used in a manner similar to swords. They can also be used as conduits for magic skills such as Fire or Blizzard. They are very effective weapons against Heartless and Nobodies.

The main power of the Keyblade, however, is the ability to lock (and possibly unlock) the Keyholes of worlds, sealing away its heart. As an offset of this ability, the Keyblade can also be used to lock or unlock any lock, allowing the user to access any sealed interior, whether it be a locked room, gate or treasure chest.

In addition to its base powers, Sora's Keyblade in particular has been shown to be customizable via keychains he finds or receives along his journey, giving it additional abilities or boosting its offensive and/or magical powers. Furthermore, the Keyblade's appearance changes depending on what keychain is attached. Sora is the only wielder in the series to have altered his Keyblade with different keychains.

Dual Wielding
At certain points in the series, characters are able to use two Keyblades simultaneously. Roxas is the only individual to demonstrate this ability naturally. Sora is able to produce a second Keyblade using the Sync Blade ability while in the Valor, Master, or Final Drive Forms. He also used Riku's keyblade alongside his to execute a finishing move upon Xemnas in the final battle of Kingdom Hearts II. The power to possess two Keyblades at once has not been expanded upon or explained in the series at all, so it is unknown if other wielders are able to possess two.

Appearance
All Keyblades shown have the same distinguishing features that are modeled after a key: a guard with a handle, a hilt and a head at the end of the shaft that resembles the teeth of a key. All Keyblades also have some theme, usually a World or the person who bears the Keyblade, that will reflect in its appearance.

Seen below are the Keyblades main character Sora has used throughout Kingdom Hearts and Kingdom Hearts II, via attaching keychains from various worlds and people to alter his main blade's appearance.

Acquisition
It is said that a Keyblade will choose its own master, not the reverse. So far, it is shown only one with a strong heart can wield a Keyblade, be they good or evil. Keyblades can also change masters when they detect a stronger heart or a change of heart; for example, the Kingdom Key jumps to Riku at one point in Kingdom Hearts because he has the stronger heart, but after he is granted powers of Darkness by Maleficent, it chooses to return to Sora as it does not work for those with darkness in their hearts. This is because that specific Keyblade is the Keyblade of Light.

A known exception to the rule is Roxas, a Nobody who is able to use Keyblades despite not having a heart by definition of a Nobody. The most logical explanation for this is that because Roxas is the Nobody of Sora, who is able to use the Keyblade naturally, he inherited this ability from Sora. This would mean that Keyblades aren't just drawn to those with strong hearts, but must also have some connection with one's body and/or soul. Xion, the mysterious fourteenth member of Organization XIII in Kingdom Hearts 358/2 Days, is also shown in promotional shots to be wielding what appears to be the Kingdom Key, even though it is assumed that she is a Nobody like Roxas.

History
The origins of the Keyblades is, thus far, unclear. Each game references the special qualities of the wielders and some give vague explanations of the Keyblades' power, but none have shown where the Keyblades originated, how they are created, or what their original purpose might have been. One legend mentioned in Kingdom Hearts states that the Keyblade master saved the world, while another said he wrought chaos and destruction upon it. While this legend may appear to contradict itself, it can be assumed that the Keyblade master chooses how to use the Keyblade, whether for good or evil. The keyblade is known throughout the worlds as a unique and powerful weapon. In Atlantica, King Triton and Ursula both knew of the keyblade and its powers. King Triton did not want Sora to seal the keyhole because he thought that the keyblade would send his world into chaos, while Ursula knew that it gave the ability for the one who wields it to travel around the worlds.

One of the pieces of text in the hidden ending video for Kingdom Hearts II, "The Gathering", mentions a "Keyblade War". Hundreds of Keyblades can be seen stuck into the ground during this cinematic, as well as the extended "Birth By Sleep" cinematic at the end of Final Mix+. It should be noted that none of these seemingly abandoned Keyblades, nor those of protagonists Aqua, Terra or Ven, have keychains, although the significance of this detail is currently unknown as the history of the Keyblades.

Keyblade Wielders
For a complete list, see: List of Known Keyblade Wielders

Sora
The most prominent character in the series to use a Keyblade is Sora, who has used them successfully in the two main games in the series(with one minor incident). His ability to use and possess the Keyblade was a major plot point in the first game. He has used more Keyblades than anyone else in the Kingdom Hearts series; the variations are created through various keychains attached to his original blade, the Kingdom Key.

Theories
There are many theories surrounding the Keyblade that include what a Keyblade's purpose is, how a Keyblade is formed, and the significance of a keychain. Various complications occur when the crossroad with Keyblades with no keychain from previous games were shown.

While the origins of the keychains are a mystery, there are many theories to its purpose, such as that they are charms that summon the keyblade to its host.

Another theory speculates that when one creates a keyblade, as Xehanort did by the use of Riku, there is no keychain with it. That would indicate that the keyblade can then be created either by the person or from the person, as Xehanort's Keyblade was created from the Princesses of Heart, or how Riku's Keyblade was created from the Soul Eater, which was forged out of his own darkness. The keyblade can then be seen as an extension of it's creator, and that a Keyblade's power comes from its creator alone. Therefore, when a keyblade needs to be passed to a person, a keychain is formed to be a power source to give the keyblade lasting energy as well as its power and form. This could possibly explain why some keyblades do not have keychains such as Terra, Ven and Aqua's Keyblade. It could also show how Riku got his Way to the Dawn Keyblade as well. This would mean each keyblade has its own unique power and form if keyblades were created by certain individuals, because people are unique and the same could be said for Keyblades. This could explain why the moogles are able to synthesize the Ultima Weapon.

Yet another theory is that the Keychain is a physical embodiment of one's memories, which would explain why Sora got new Keyblades for every world, as well as why he is able to receive them as gifts from friends (eg. Oathkeeper, Gullwing). It also explains Roxas's Keyblades; because Roxas is Sora's Nobody, Roxas's Oblivion represents Sora's memory of Riku, and his Oathkeeper represents Sora's memory of Kairi.

Another theory is that Keyblades without Keychains are merely "apprentice" Keyblades, which explains why Terra, Aqua and Ven's Keyblade's do not possess Keychains, while Master Xehanort's and "Dark Soldier's" do.

According to some fans, the Keyblade or it's creator is also the mysterious voice which guides Sora and Roxas in the beginning and ending of both games.

Trivia

 * The teeth of the key on the Kingdom Key is negative space identical to the necklace that Sora wears and the keychain on Oblivion.
 * It is unknown if Sora still possess his other Keyblades from Kingdom Hearts in Kingdom Hearts II.