Birth by Sleep: Final Mix > Secret Episode
Birth by Sleep: Final Mix actually features an entire new story to play as Aqua inside the Realm of Darkness after the events in the Final Episode. The Secret Episode, also referred to as Zero Point, features new fights against the Heartless that inhabit the Realm of Darkness, including the mysterious Red Eye, and also unlocks a new secret ending at the end of the credits.
After the events in the Final Episode, Aqua finds herself inside the Realm of Darkness, wandering through as she searches for a way out. After the introductory cutscene, take a few steps forward, use the Save Point if you want, and make your way toward the swirling black puddle just a little bit up the path. This will trigger a mandatory fight against a group of Neoshadow and Shadow Heartless, which are not very difficult to take care of. When the fight is over, the barrier drops and you can proceed.
The major goal of this area is to go around to different sections, destroying dark puddles wherever you find them by defeating the Heartless that come from them. By doing this, certain dark puddles will open new paths after they are destroyed, allowing you to progress further in the map. The dark orbs scattered throughout the area can be used to launch you up to higher places so reach more dark puddles and reach new areas.
Eventually you’ll reach an area high up with several dark areas infested with Heartless. You’ll also notice a large area sealed off by a dark force field nearby. Naturally, if you defeat all the Heartless in the dark areas, the dark force field drops, allowing you to continue. Enter the once-blocked area and you’ll be forced into a fight with large amounts of Heartless. Take them all out to advance to the next area. Use the Save Point when it’s available. Once you move on, there’s no going back.
Hunter of the Dark
Enter the area beyond the Save Point and you’ll be immediately dropped into a rather difficult boss fight.
Red Eye is a two-stage fight, but it’s difficult enough making it past stage one. Red Eye will lurk in the darkness surrounding the arena. Suddenly, the view will shift to his perspective as he closes in on Aqua. You’ll have to be quick to avoid being run over during its charge attack. After the point of view shifts back to normal, pelt Red Eye with a few hits before it vanishes into the dark again. This segment will repeat several times until you know its HP down a bar or two. Then the real fight begins!
The darkness is illuminated and Red Eye’s true form is revealed. It’ll attack with spinning slash attacks and spit dark energy balls at you, but all of these attacks can be avoided by using the Cartwheel Command to dodge. Keep hitting Red Eye when it stops briefly, then go back on defense as it goes back on offense. Keep this pattern up until Red Eye’s HP drops to below half.
Now Red Eye really switches up its strategy. It’ll move a lot more frequently, but when it does, it will now leave behind several multicolored silhouettes that can attack you as well until its combo is finished. Then the silhouettes merge back into the one body until Red Eye moves again. Also, watch out for its whirlwind attack in which the silhouettes will rush at you from within the swirling vortex they use to surround you. Use the Cartwheel Command to evade these attacks and continue hammering Red Eye when possible to finish it off for good.