The Other Key
As the two groups fight, Riku mocks Sora and his supposed "strength of heart". Eventually, Riku flees deeper into the castle. Now alone, Riku reveals that his heart feels as if it were to explode. A cloaked figure appears, encouraging Riku to be completely consumed by the darkness.
Meanwhile, Maleficent stands before the world's Keyhole, along with the sleeping Seven Princesses of Heart. Despite this gathering, the Keyhole refuses to open. Riku then appears, garbed in a much darker attire and wielding a new Keyblade, explaining that as long as Kairi's heart is missing, she will not be able to contribute her power. Riku continues, saying that unlocking this Keyhole of darkness will allow the Heartless to overrun this world. Maleficent laughs, claiming the darkness holds no power over her. Instead, it is she who will use the power of darkness to rule all the worlds. Riku smirks, then quickly releases Maleficent's heart with his Keyblade. Just as Sora, Donald, and Goofy arrive, they see Maleficent disappear into darkness.
Sora comments that the Riku before them isn't really Riku. When asked who he is, Riku introduces himself as Ansem, the Seeker of Darkness. Sora orders Ansem to free Riku's heart. The possessed Riku points at Sora, the latter falling to his knees. Riku explains that Sora must first release Kairi's heart from within him. Riku walks towards Sora, shortly stopping before him. As Riku summons his Keyblade to release Kairi's heart, the darkness inside the Keyhole beings to grow. Sora is unable to seal the Keyhole, as the Keyhole is still incomplete. Before leaving through a portal of darkness, Riku tosses Sora his own Keyblade, claiming that Sora must release his heart. Despite Goofy and Donald's worry, Sora immediately unlocks his own heart, resulting in his physical self to fade away to light.
(Numbers indicate order of appearance.)
Items of note
Kingdom Hearts references