The Secret Dungeon
This dungeon opens up after the third time you visit Hollow Bastion, just before the final fight with Demyx. Reach it by visiting the Postern and jumping into the hole in the wall. You start in the Depths. Leap up and claim the AP Boost (23/45). Drop down to the bottom and pick up the Power Crystal (24/45) amidst the Heartless. At the opposite end collect a Frost Crystal (25/45) then climb up the side and leap into the alcove to collect Twilight #6. From here return to the entrance of this room, leaping up to Twilight #9 on the ledge.
Cross over to the exit in the distance, then continue the ascent coming back. Collect the Found Illusion (26/45) here then Glide across to the AP Boost (27/45) at the far end. Exit into the Mineshaft where you can pick up the Deep Recollection Map (41/45). Walk around and pick up the AP Boost (39/45) then use Quick Run Lv3 to shoot through the pipes and collect Twilight #30 . You are forced into a fight, just eliminate your enemies.
Deeper and Deeper
Exit to the Depths again, just a different location. Pick up the Remembrance Gem (28/45) then head into the Mining Area. Grab the Serenity Gem (33/45) to the left then smash the three valves here. Return to the Mineshaft and use the door to reach a different part of the Mining Area. Ride the pillar up then jump over to the wind vortex. Use "Ride the Wind" to get way up high then Glide back towards the entrance to spot a large pillar that takes you up to Twilight #16. Leap from here to an AP Boost (30/45) then cross the pipes to collect Twilight #27 .
Continue along this path to find a Serenity Crystal (31/45) across from the exit door. Head into the corner to snag A Map of the Area (34/45) and a Found Illusion (32/45). Go under this platform to pick up Twilight #19 then jump down to the center to find a Serenity Gem (29/45). Glide over to Twilight #14 then exit at the top door. Pick up the Power Boost (42/45) The next part is a bit tough because you must Quick Run into the first set of pipes and just as you fall under them, use Aerial Dodge to cross and pick up Twilight #1 . Defeat the Heartless that lock you in then enter the Engine Chamber.
Almost there! Cross the conveyor belts then up the step blocks to a large belt. Pick up the Serenity Crystal (35/45) from the corner then follow the conveyor to the end, just don’t fall off. Dodge the steam and leap up at the far end to find a Remembrance Crystal (36/45). Continue the climb up to the exit door, but go past it and Glide to an AP Boost (37/45). From here leap left and Glide to a small platform with a Found Illusion (38/45). Enter the Mineshaft again, picking up a Magic Boost (43/45).
Leap from the edge and Glide all the way around, picking up Twilight #32, Twilight #36, and Twilight #40 enroute to an AP Boost (40/45) and the exit. In this area you’ll have to fight through a lot of Nobodies; seriously it is an absurd amount. At the end you’ll receive a bonus though!
BONUS : Item Slot (Sora), HP +4 (Donald), HP +5 (Goofy)
In the Garden of Assemblage you can pick up the final two items, the Garden of Assemblage Map (44/45) and a Lost Illusion (45/45). "Analyze" the device to reveal Data Replicas that may be fought. This also opens up an exit right outside the Postern so you don’t have to trek all the way back through. After you beat all Data Replicas of Organization XIII a chest appears near the terminal. It has the Non-Existent Proof (46/46) inside. This will put a crown on Sora’s head if you haven’t obtained any other Proof yet, or if you have a crown already, then the color will change.
Replica Data Rewards
For defeating the Replica Data you’ll receive additional items which will aid in increasing Sora’s stats and upgrading equipment. The Replica Data are the same, just at level 80.
I - Xemnas’s Replica Data is fought at Memory’s Contortion and the final arena, and relinquishes a Power Boost. Sora’s equipment, items, and abilities can be reconfigured between the two rounds. While Donald and Goofy are not available as allies, Riku joins the party at the second battle.
II - Xigbar’s Replica Data is fought at the Hall of Empty Melodies, and relinquishes a Defense Boost.
III - Xaldin’s Replica Data is fought at the Bridge, and relinquishes a Defense Boost. Donald and Goofy are available as allies, but the Beast is not.
IV - Vexen’s Replica Data is fought at the 11th Floor facsimile of The Old Mansion, and relinquishes a Lost Illusion.
V - Lexaeus’s Replica Data is fought at the Station of Remembrance, and relinquishes a Lost Illusion.
VI - Zexion’s Replica Data is fought at the 3rd Basement facsimile of the Seashore, and relinquishes a Lost Illusion.
VII - Saïx’s Replica Data is fought at the Addled Impasse, and relinquishes a Defense Boost.
VIII - Axel’s Replica Data is fought at the Basement, and relinquishes a Magic Boost. Donald, Goofy, and Roxas are not available as allies.
IX - Demyx’s Replica Data is fought at the Castle Gate, and relinquishes an AP Boost.
X - Luxord’s Replica Data is fought at the Havoc’s Divide, and relinquishes an AP Boost. Donald and Goofy are not available as allies.
XI - Marluxia’s Replica Data is fought at the Station of Oblivion, and relinquishes a Lost Illusion.
XII - Larxene’s Replica Data is fought at the Station of Remembrance, and relinquishes a Lost Illusion.
XIII - Roxas’s Replica Data is fought at the Station of Awakening, and relinquishes a Magic Boost. Donald and Goofy are not available as allies.