A Fragmentary Passage

"A Fragmentary Passage", also referred to as the "Secret Episode" (シークレットエピソード), is a secret ending exclusive to Kingdom Hearts Birth by Sleep Final Mix. Unlike other secret endings, "A Fragmentary Passage" is playable, much like the Final Episode, rather than simply watching from the view of a bystander.

In "A Fragmentary Passage", Aqua is playable, the main setting being the Realm of Darkness, where Aqua fights several Pureblood Heartless while wielding Eraqus's Master's Defender.

Synopsis
The secret ending opens with Sora sleeping on the Destiny Islands, with Kairi approaching him to wake up, mirroring the opening to Kingdom Hearts.

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Aqua is seen walking up to the cloaked Ansem the Wise, as seen in "Blank Points".

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Clips from Blank Points are then shown rewinding, including the scenes between Braig and Xehanort, Terra and Master Xehanort, and Aqua in the Realm of Darkness.

Zero point

Aqua is then seen walking on through the Realm of Darkness, but stops when the realm starts to shake. A portal of light appears above her, and a jet of darkness appears flowing directly into the portal. She clutches her Wayfinder to her chest as the light within it starts to fade, and decides to investigate this strange occurrence.

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While exploring further, Aqua encounters a mysterious enemy. Upon first noticing it, Aqua draws her Keyblade, but it quickly knocks her down. Aqua recovers and strikes it before it can attack her further, realizing it is not an Unversed. She then fights it and emerges victorious.

Aqua then spots the remnants of the Castle of Dreams, which has been swallowed by the Realm of Darkness. She looks in horror at the castle ahead of her.

A fragmentary passage

Mickey Mouse is then seen walking through the Realm of Darkness with the Star Seeker Keyblade, Riku is seen atop Neverland's clock tower with Kairi in his arms, Donald Duck and Goofy are seen wandering through Traverse Town, Kairi, in her Kingdom Hearts II outfit, is seen with Hayner, Pence, and Olette, looking at the Old Mansion, and DiZ is seen in front of a sleeping Sora recording data. Roxas and Xion are seen in Twilight Town with a blindfolded Riku looking down on them. Finally, a young Master Xehanort is seen on the Destiny Islands beach, then turns towards the camera, transforming into Ansem, Seeker of Darkness, then Xemnas, then a flurry of clips showing the elder Master Xehanort. Sora is then seen sitting on the paopu tree in Destiny Islands, but disappears. Ventus is then seen in Chamber of Waking, but also disappears, leaving the room empty.

Birth by Sleep -Volume Two-

Unlock criteria
The conditions that must be met in order to unlock "Blank Points" and "A Fragmentary Passage" depend upon the difficulty mode the player has chosen for each character's story. It's always required to unlock the Final Episode first by collecting all Xehanort's Reports.
 * Beginner Mode - Complete the Sticker Album with Terra, Aqua, and Ventus and obtain the Keyslinger, In the Munny, Power Walker and Clockwork trophy at most.
 * Standard Mode - Complete the Sticker Album with Terra, Aqua, and Ventus and obtain the Keyslinger trophy.
 * Proud Mode - Complete the Sticker Album with Terra, Aqua, and Ventus.
 * Critical Mode - Complete the Final Episode.

Notes and references
A fragmentary passage