This part of the game is unlocked when you defeat Marluxia, and create a save game afterwards. You play as Riku as he travels from the basement of Castle Oblivion. After the opening scene you'll receive Hollow Bastion World Card and arrive in B12F.
Riku gains HP (Health Points), AP (Attack Points), and DP (Dark Points) when he levels up instead of HP, CP (Card Points), and Sleights. Riku also cannot change his deck, instead the deck changes on each floor. Riku starts in the basement of Castle Oblivion instead of the first floor, but he still moves up the levels of the castle. Riku can reload his deck instantly and the counter never increases.