Door to Darkness

The Door to Darkness 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 Xehanort's Heartless multiple times in the quote "The Door is open!", which is also spoken by Riku when the Destiny Islands are swallowed by the darkness at the beginning of Sora's adventures.

Kingdom Hearts
After the battle with the World of Chaos, Xehanort's Heartless 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 II
In 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 Duck, 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.

Towards the end of the game in The World That Never Was, after Xemnas had absorbed the power of Kingdom Hearts, a similar door appears, allowing access to the Realm of Darkness. Mickey states that the door was indeed a gift from the worlds, who had accepted Sora as their keeper. Sora and the others all enter through the door, but after Xemnas's interference via flying buildings, it is narrowed down to just Sora, Riku, Donald and Goofy. It is here that the last battles with Xemnas are fought.

Trivia

 * In all other versions of the Kingdom Hearts game, the term "DTD" and its English meaning, "Door to Darkness" are used by Sora rather than King Mickey. The term will be later translated by King Mickey upon his entry to Ansem's study.
 * In Kingdom Hearts Re:Chain of Memories the Final battle of Riku agaisnt Xehanort's Heartless happen in a similar place to the one where the Door to Darkness is