Final Keyhole

The Final Keyhole is the last Keyhole seen in Kingdom Hearts. It is in Hollow Bastion and requires the seven hearts of the Princesses of Heart to unlock the Keyhole and open the Door to Darkness.

Kingdom Hearts
Maleficent and her accomplices begin to gather the Princesses of Heart to gain the powers of darkness and open the door to Kingdom Hearts. Maleficent recruits Riku to her cause, telling him that he can rescue Kairi's heart with the power of Kingdom Hearts.

By the time the Princesses are all gathered, Maleficent and Riku are the only ones remaining. However, the Keyhole remains incomplete while Kairi is still missing her heart. After finally defeating Maleficent, Sora confronts Riku, who has been possessed by Ansem, Seeker of Darkness. Ansem reveals that Sora is carrying Kairi's heart within him, and he attempts to release it with a Dark Keyblade. Sora fights him off, but he uses the Keyblade on himself in order to save Kairi, freeing the hearts of the other Princesses and losing his own in the process. Kairi is revived, completing the Final Keyhole.

When the Keyhole is revealed, a wave of darkness and several powerful Heartless are released. These are the Phantom, Kurt Zisa, and the Behemoth. When Sora returns to Hollow Bastion, he defeats the Behemoth and locks the Keyhole. However, Ansem has already departed for the End of the World and Kingdom Hearts.

Kingdom Hearts II
The Final Keyhole appears in Sora's memories as Roxas dreams about them.

Kingdom Hearts 358/2 Days
The Final Keyhole appears in Xion's dreams as one of Sora's memories.