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Keyblade Master is the title given to exceptional Keyblade wielders who pass the Mark of Mastery exam in the Kingdom Hearts series. There are a total of six known Keyblade Masters: Master Eraqus, Master Xehanort, Yen Sid, Aqua, Mickey Mouse, and Riku. Currently, three of the six remain active: Mickey, Aqua, and Riku. Master Xehanort previously lost his memories, becoming the younger Terra-Xehanort, but was restored following the destruction of his Heartless and his Nobody. Yen Sid retired his title and passed his Keyblade down to Mickey, Eraqus was slain by Master Xehanort, who himself was slain by Sora.
A Master is not necessarily the most powerful Keyblade wielders as Sora and Terra both have defeated Master Xehanort in his original incarnation and possessing the more powerful and young vessel of Terra's body. Master Xehanort, who himself was one of the most powerful Keyblade wielders alive, has lost multiple times to non Masters. Like Mickey and Riku, Sora also has the Power of Waking, one of the most powerful abilities a Keyblade wielder or Keyblade Master can possess, despite not having the title of Master.
The Keyblade Master, upon passing the Mark of Mastery exam, is given the choice to either travel to many worlds and explore his or her horizons, or to succeed his or her predecessor and take refuge in his or her own world to protect it from malevolent forces and later take on more apprentices.
In the age of fairy tales, the Master of Masters, after studying the χ-blade, crafted Keyblades from the hearts of others, inventing Keyblades. He trained six other Keyblade Masters. One was chosen to carry the Gazing Eye Keyblade, Luxu, which could see the future, and Luxu was to repeat this process of training an apprentice to inherit the blade. This allowed the eye to see into the future, allowing for the Book of Prophecies the Master of Masters wrote to exist. In this time did the first Keyblade War unfold. From these masters did the idea to train new masters out of apprentices arise. The aftermath of the battle destroyed the light, and the χ-blade, with only the light in children's hearts keeping the world from being completely overrun. The true Kingdom Hearts was lost to darkness following the clash. This allowed for worlds to exist, though separate.
Long before the events of Kingdom Hearts Birth by Sleep, two young men, Eraqus and Xehanort, were both tested for the Mark of Mastery by their Master. They both passed, but Eraqus was chosen as their Master's successor.
Xehanort abused his young apprentice by forcing him to fight creatures of the dark in order to strengthen his darkness so both his light and darkness could be equal, which would result in the χ-blade, a legendary Keyblade which allows its user to obtain Kingdom Hearts, the heart of all worlds. Ventus refused to fight the creatures, because he knew that he was not strong enough to take them on. The creatures attacked Ventus, knocking him unconscious. Xehanort approached his young apprentice and used his Keyblade to unlock his heart, and extract the darkness from within, resulting in the creation of a being with a heart of pure darkness, which Xehanort named Vanitas. This process left Ventus's heart fractured.
Xehanort brought Ventus's nearly lifeless body to his childhood home, Destiny Islands, so he could succumb in peace. However, Ventus's fractured heart was met by a newly born heart of another, and the two became one; just until Ventus's heart was strong enough to survive on it's own. Ventus then summoned his Wayward Wind Keyblade, which completely surprised Xehanort.
Ventus was then taken by Xehanort to his old fellow pupil, Eraqus. He asked Eraqus to take him in, knowing that Eraqus would strengthen his heart of light, while he continued to train Vanitas, to strengthen his heart of darkness, so that one day the two hearts would be able become one again and forge the χ-blade.
At an unknown amount of time before the events of Kingdom Hearts Birth by Sleep, Yen Sid had already trained and took on the title of an Keyblade Master. It is unclear if he had trained simultaneously along with Eraqus's and Xehanort's Master, or was their Master or a colleague of their Master. By the events of Kingdom Hearts Birth by Sleep, he had retired officially from the position as a Master, though he took his former apprentice, Mickey Mouse as his apprentice for more training.
After many years of training, Terra and Aqua were ready to take the Mark of Mastery exam. Unfortunately for Terra, he fails, while Aqua passes and is chosen as the successor of Eraqus. Terra could not control the darkness in his heart, a trait that Master Eraqus did not see fitting for the title of Master.
Eraqus informs Aqua of her new duties as a Keyblade Master, including a "secret" in particular that she should know; as his successor, if anything were to happen to him, and the darkness plundered the world, he wanted her to take his Keyblade and lock the world away. He informs her that generations of Keyblade Masters have been entrusted with keeping the world safe, because the light and darkness were balanced there, and there were people who might want to abuse such a neutral powered world. Their predecessors devised a "trick" that allowed the Keyblade to transform the world, so that all who visit the world will be lost to oblivion, and none ever would be able to solve its mystery, except for the one who transformed it.
Meanwhile, retired Master Yen Sid had allowed his former apprentice, Mickey, to come back to him for more training. Yen Sid discovers that the worlds were being threatened by creatures that fed off negativity, which he called the Unversed, and that the Princesses of Heart were in danger from them. Mickey sees this as an opportunity to prove that he was Master material, so he steals his former Master's Star Shard, and leaves to investigate.
Yen Sid informs Eraqus of the situation and he calls forth Terra and Aqua to investigate. He tells Terra that if he performs well on this journey, he might reconsider making Terra a Keyblade Master, but he must not rely heavily on his darkness. However, Master Xehanort manipulates Terra into succumbing to darkness and possessed him, transforming him into Terra-Xehanort.
Much later, Aqua returns to the Land of Departure with Ventus, who lost his heart. She sees that it was completely destroyed by the darkness. She finds the fallen Master Eraqus's Keyblade lying in the remains and takes it. She brings Ventus into the castle and places him in one of the thrones. Remembering the "trick" the Master told her about, she uses his Keyblade to completely transform the world. She promises Ventus to return with Terra to wake him up, and leaves him in the newly-formed Chamber of Waking. As she leaves the castle, she turns to see the new world she created, Castle Oblivion. She then leaves to search for the missing Terra, but following a brutal fight with Xehanort, Aqua ends up trapped in the Realm of Darkness.
Mickey and Yen Sid converse about the revival of Xehanort. Knowing that Xehanort's revival will lead to the return of Master Xehanort, Yen Sid commands Mickey to summon both Sora and Riku to him, so that they can be tested for the Mark of Mastery in order to become Keyblade Masters themselves and finish Master Xehanort once and for all.
Powers and abilities
All Keyblade Masters share the ability to extract the hearts of themselves or others, this is shown when Master Xehanort extracts the hearts of Aurora and himself. They can also extract part of the heart as well, for instance, Xehanort extracts the darkness from Ventus's heart, and creates Vanitas.
Keyblade Masters also have the ability to perform the Keyblade Inheritance Ceremony, a trial in which a person is chosen to inherit the power of the Keyblade by clutching the Keyblade in their hand and allowing it to accept them. This is shown when Terra performs it on Riku, and when it is accidentally performed on Kairi when she grasps Aqua's Keyblade. Although, Terra does not have the title of Master, he is at the level of one, therefore he is able to perform this rite.
In addition to performing the Keyblade Inheritance Ceremony, Masters can conduct the Mark of Mastery exam upon potential Keyblade Masters. Although Yen Sid is a retired Master, he is still able to conduct the exam.
Also, Aqua is able to sense that the ceremony has been performed on Riku, although it is not known whether all Keyblade wielders can sense this, or if it is a Master's ability.