When Sora, Donald, and Goofy return to Castle Oblivion from Twilight Town, they are greeted by an unlikely visitor. It's Riku, and, although Vexen was just killed by Axel, he still seems to be dark. Sora tells Riku that Vexen is dead and he's free to turn away from the darkness, but Riku doesn't budge. He says he made a promise to Naminé, which confuses Sora - he was the one that made a promise to Naminé, not Riku. It's not long before both pull out the same good luck charm, and both claim it was given to them by Naminé. Riku gets angry at Sora and calls him a liar, and then becomes hostile. Naturally, you have to fight back. Like always, there is no real strategy to defeating Riku. All you have to do is dodge his attacks and bring in your own combos. This Riku battle may be pretty annoying mostly due to the fact that Riku is constantly card breaking your attacks. Use the Cloud Summon Cards and you'll be fine.
After defeating Riku, another world will become available. A cutscene will show Sora becoming angered with Donald and Goofy when they question Sora's purpose for finding Naminé. He leaves the two and storms off on his own. You'll lose the Donald Cards and Goofy Cards. Now go to the world that has just shown up, Destiny Islands.
When Sora arrives, he's greeted by Selphie, Wakka, and Tidus. After the conversation, they leave. Sora remembers Destiny Islands as the place he and Naminé used to play together, though he still is a bit confused. In this room, kill all of the Heartless and strike the door, then use a card with a value of 1 or more. In Room 2, kill all of the Heartless and strike the door. Synthesize using a Moment's Reprieve Card if you have one. Strike the door to the east using a card with a value of 2 or more, a card with a value of 8 or more, and the Key of Beginnings. Riku and Sora have a conversation. After the cutscene, you'll be back in Room 1. Go back to Room 3 and strike the north door. Synthesize using a card with a value of 7 or higher. In Room 4, kill all of the Heartless and strike the door. Synthesize using a card with a value of 8 or more. In Room 5, kill all of the Heartless and strike the door. Synthesize using a card with a value of 9 or higher. In Room 6, strike the door after clearing the room of Heartless and synthesize using a Gimmick Card. In Room 7, strike the door and synthesize cards to reach the value of 50 and the Key of Guidance.
Darkside is an easy boss battle frustrating only because of its large amount of health. Its health reaches a fourth bar, making it a rather time-consuming battle. Use Sleights and Summon Cards as well as combos to finish this boss off. Card break if you want to, but it's really not necessary as long as you have the right Item Cards. Darkside's attacks include:
- When Darkside brings his fist down and slams it onto the ground, that's his way to summon Shadows. Get rid of these quickly so they don't get in the way. Although they're small, they can easily get annoying.
- Again, Darkside slams his hand down onto the ground. This time, however, instead of summoning more Heartless, a shock wave is made by the impact. An attack that is relatively easy to avoid.
- A large sphere is emitted by Darkside. Another attack that is easily dodged.
- Darkside emits a large sphere once again, except this time, it doesn't aim for Sora. It aims upward for the sky, and splits into many smaller spheres. They rain down on Sora, making this a little hard to dodge. This is the attack of the battle to avoid.
Once you've defeated Darkside, a cutscene will have Naminé explain to Sora that everything he thinks - including his constant thought that he has to protect Naminé is false. Unfortunately, this only further confuses Sora. He starts to remember Kairi, but still doesn't quite remember her yet. All he can think of is Naminé. After the cutscene, you'll get the Oathkeeper. Now leave Destiny Islands.