Door to Darkness

The Door to Darkness, also known as Final Door in Kingdom Hearts, is an object from the Kingdom Hearts games. A trademark of the series, the Door is a tall, white door first seen in Kingdom Hearts in the End of the World. Beyond the door is the Realm of Darkness where Kingdom Hearts, the heart of all worlds, resides. It is referred to by Ansem, Seeker of Darkness multiple times in the quote "The Door is open!", which is also spoken by Riku when Destiny Islands is swallowed by the darkness at the beginning of Sora's adventures.

Kingdom Hearts
After the battle with the World of Chaos, Ansem, Seeker of Darkness opens the door and asks that Kingdom Hearts fill him with the power of darkness. But to his horror, Kingdom Hearts is revealed to be light and it destroys him. Afterwards, Sora, Donald, Goofy, and the restored Riku attempt to close the door, but their efforts are futile. However, King Mickey appears, and they manage to close the door. As they do so, the King assures Sora, Donald, and Goofy that there will always be a door to the light, and Sora and Mickey then use their respective Keyblades to seal the Door to Darkness forever.

Kingdom Hearts Re:Chain of Memories
In Kingdom Hearts Re:Chain of Memories, the final battle of Riku against Ansem happen in a similar place to the one where the Door to Darkness is.

Kingdom Hearts II
The Door to Darkness appeared in Sora's memories when Roxas was dreaming about them. It is also connected to Space Paranoids. Tron needs a password to access the "D.T.D.", and when Sora, Donald, and Goofy return to Radiant Garden, King Mickey reveals that the "D.T.D." refers to the Door to Darkness. And since the seven Princesses of Heart were required to open the Door to Darkness, the names of the Princesses were the password. In order to prevent the MCP from accessing the data within, Tron changes the password to the names of Sora, Donald, and Goofy.

After Xemnas escapes Sora and his friends at the Altar of Naught by merging with his Kingdom Hearts, a door similar to the Door To Darkness appears, leading to the World of Nothingness Xemnas has created within. Mickey states that the door was a gift from the worlds, who have accepted Sora as their keeper. Sora and the others pass through the door, and after they seemingly defeat Xemnas they return once more to the Altar of Naught, where Naminé opens a final Corridor of Darkness to Destiny Islands. Just as Kairi passes through the portal, Xemnas returns on his gigantic Nobody and attacks the tower, dragging Sora and Riku back into the World of Nothingness. This time, however, they do not pass through the door.