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 five known Keyblade Masters; Master Eraqus, Master Xehanort, Yen Sid, Aqua, and Mickey Mouse. Currently, only three remain active; Mickey, Aqua, and the newly reformed Master Xehanort. Yen Sid retired this title and passed his Keyblade down to Mickey, and Eraqus was slain by Master Xehanort.
Long before the events of Kingdom Hearts Birth by Sleep, 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 it's user to obtain Kingdom Hearts, the heart off 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 Keyblade, which completely surprised Xehanort.
Ventus was then taken by Xehanort to his old 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 give him the χ-blade.
Kingdom Hearts Birth by Sleep
After many years of training, Terra and Aqua were ready to take the Mark of Mastery exam. Unfortunately for Terra, he failed, while Aqua passed and was chosen as the successor of Eraqus. Terra could not keep the darkness in his heart in check, a trait that Master Eraqus did not see fitting for the title of Master.
Eraqus informed Aqua her new duties as a Keyblade Master, include a "secret" in particular that she should know; as his successor, if anything were to happen to him, and the darkness plundered the word, he wants her to take his Keyblade and lock the world away. He informed her that generations of Keyblade Masters have been entrusted with keeping the world safe, because the light and darkness are balanced there, and there are people who might want to abuse such a neutral powered world. Their predecessors devised a "trick" that allows 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 it's 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 discovered that the worlds were being threatened by creatures that feed off of negativity, which he called the Unversed, and that the Princesses of Heart were in danger from them. Mickey saw this as an opportunity to prove that he is Master material, so he stole his former Master's Star Shard, and left to investigate.
Yen Sid informed Eraqus of the situation and he called forth Terra and Aqua to investigate. He told Terra, that if he performs well on this journey, he might reconsider to make Terra Master, but he must not rely heavily on his darkness.
Later, Aqua returns to the Land of Departure with an unconscious heartless Ventus. 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. She remembers the "trick" her Master told her about, and uses his Keyblade to completely transform the world. She tells Ventus that she'll 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 find a missing Terra.
After Kingdom Hearts Birth by Sleep
Sometime after the events of Kingdom Hearts Birth by Sleep, Mickey was finally given the title of Keyblade Master by Yen Sid.
Knowing that Master Xehanort has returned and that he may not be alone, Yen Sid commands Mickey to summon both Sora and Riku to him, so that they can be tested for the Mark of Mastery.
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 heart 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 on the level of one, therefore he is able to perform this rite.
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.